High energy density redox flow device

ABSTRACT

Redox flow devices are described including a positive electrode current collector, a negative electrode current collector, and an ion-permeable membrane separating said positive and negative current collectors, positioned and arranged to define a positive electroactive zone and a negative electroactive zone; wherein at least one of said positive and negative electroactive zone comprises a flowable semi-solid composition comprising ion storage compound particles capable of taking up or releasing said ions during operation of the cell, and wherein the ion storage compound particles have a polydisperse size distribution in which the finest particles present in at least 5 vol % of the total volume, is at least a factor of 5 smaller than the largest particles present in at least 5 vol % of the total volume.

RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/970,773, filed Dec. 16, 2010, which patent wasgranted and issued as U.S. Pat. No. 8,722,226 on May 13, 2014, which isa continuation-in-part application of U.S. patent application Ser. No.12/484,113, filed Jun. 12, 2009, which claims priority under 35 U.S.C.§119(e) to U.S. Provisional Patent Application Ser. No. 61/060,972,filed Jun. 12, 2008 and U.S. Provisional Patent Application Ser. No.61/175,741, filed May 5, 2009, all entitled “High Energy Density RedoxFlow Device,” the contents of which are hereby incorporated by referencein their entirety.

This application also is a continuation application of U.S. patentapplication Ser. No. 14/002,304 filed Feb. 11, 2014, which is a 371application of International Application PCT/US11/65615, filed Dec. 16,2011, which application claims priority to U.S. patent application Ser.No. 12/970,773, filed Dec. 16, 2010, all entitled “High Energy DensityRedox Flow Device,” the contents of which are incorporated in theirentirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberDE-FC26-05NT42403 awarded by the Department of Energy. The governmenthas certain rights in this invention.

INCORPORATION BY REFERENCE

All patents, patent applications and documents cited herein are herebyincorporated by reference in their entirety for all purposes.

BACKGROUND

A battery stores electrochemical energy by separating an ion source andan ion sink at differing ion electrochemical potential. A difference inelectrochemical potential produces a voltage difference between thepositive and negative electrodes; this voltage difference will producean electric current if the electrodes are connected by a conductiveelement. In a battery, the negative electrode and positive electrode areconnected by two conductive elements in parallel. The external elementconducts electrons only, and the internal element (electrolyte) conductsions only. Because a charge imbalance cannot be sustained between thenegative electrode and positive electrode, these two flow streams supplyions and electrons at the same rate. In operation, the electroniccurrent can be used to drive an external device. A rechargeable batterycan be recharged by application of an opposing voltage difference thatdrives electronic current and ionic current in an opposite direction asthat of a discharging battery in service. Thus, the active materials ofrechargeable batteries need to be able to accept and provide ions.Increased electrochemical potentials produce larger voltage differencesthe cathode and anode, and increased voltage differences increase theelectrochemically stored energy per unit mass of the device. Forhigh-power devices, the ionic sources and sinks are connected to theseparator by an element with large ionic conductivity, and to thecurrent collectors with high electronic conductivity elements.

Rechargeable batteries can be constructed using static negativeelectrode/electrolyte and positive electrode/electrolyte media. In thiscase, non-energy storing elements of the device comprise a fixed volumeor mass fraction of the device; thereby decreasing the device's energyand power density. The rate at which current can be extracted is alsolimited by the distance over which cations can be conducted. Thus, powerrequirements of static cells constrain the total capacity by limitingdevice length scales.

Redox flow batteries, also known as a flow cells or redox batteries orreversible fuel cells are energy storage devices in which the positiveand negative electrode reactants are soluble metal ions in liquidsolution that are oxidized or reduced during the operation of the cell.Using two reversible redox couples, liquid state redox reactions arecarried out at the positive and negative electrodes. A redox flow celltypically has a power-generating assembly comprising at least anionically transporting membrane separating the positive and negativeelectrode reactants (also called catholyte and anolyte respectively),and positive and negative current collectors (also called electrodes)which facilitate the transfer of electrons to the external circuit butdo not participate in the redox reaction (i.e., the current collectormaterials themselves do not undergo Faradaic activity). Redox flowbatteries have been discussed by C. Ponce de Leon, A. Frias-Ferrer, J.Gonzalez-Garcia, D. A. Szantos and F. C. Walsh, “Redox Flow Batteriesfor Energy Conversion,” J. Power Sources, 160, 716 (2006), M.Bartolozzi, “Development of Redox Flow Batteries: A HistoricalBibliography,” J. Power Sources, 27, 219 (1989), and by M.Skyllas-Kazacos and F. Grossmith, “Efficient Vanadium Redox Flow Cell,”Journal of the Electrochemical Society, 134, 2950 (1987).

Differences in terminology for the components of a flow battery andthose of conventional primary or secondary batteries are herein noted.The electrode-active solutions in a flow battery are typically referredto as electrolytes, and specifically as the catholyte and anolyte, incontrast to the practice in lithium ion batteries where the electrolyteis solely the ion transport medium and does not undergo Faradaicactivity. In a flow battery, the non-electrochemically active componentsat which the redox reactions take place and electrons are transported toor from the external circuit are known as electrodes, whereas in aconventional primary or secondary battery they are known as currentcollectors.

While redox flow batteries have many attractive features, including thefact that they can be built to almost any value of total charge capacityby increasing the size of the catholyte and anolyte reservoirs, one oftheir limitations is that their energy density, being in large partdetermined by the solubility of the metal ion redox couples in liquidsolvents, is relatively low. Methods of increasing the energy density byincreasing the solubility of the ions are known, and typically involveincreasing the acidity of the electrode solutions. However, suchmeasures which may be detrimental to other aspects of the celloperation, such as by increasing corrosion of cell components, storagevessels, and associated plumbing. Furthermore, the extent to which metalion solubilities may be increased is limited.

In the field of aqueous electrolyte batteries, and specificallybatteries that utilize zinc as an electroactive material, electrolytesthat comprise a suspension of metal particles and in which thesuspension is flowed past the membrane and current collector, have beendescribed. See for example U.S. Pat. Nos. 4,126,733 and 5,368,952 andEuropean Patent EP 0330290B1. The stated purpose of such electrodes isto prevent detrimental Zn metal dendrite formation, to preventdetrimental passivation of the electrodes, or to increase the amount ofzincate that can be dissolved in the positive electrode as the celldischarges. However, the energy density of such fluidized bed batterieseven when electrolytes with a suspension of particles are used remainsrelatively low.

Thus, there remains a need for high energy-density and highpower-density energy storage devices.

SUMMARY

Redox flow energy storage devices are described in which at least one ofthe positive electrode or negative electrode-active materials mayinclude a semi-solid or a condensed ion-storing liquid reactant, and inwhich at least one of the electrode-active materials may be transportedto and from an assembly at which the electrochemical reaction occurs,producing electrical energy. By “semi-solid” it is meant that thematerial is a mixture of liquid and solid phases, for example, such as aslurry, particle suspension, colloidal suspension, emulsion, gel, ormicelle. “Condensed ion-storing liquid” or “condensed liquid” means thatthe liquid is not merely a solvent as it is in the case of an aqueousflow cell catholyte or anolyte, but rather, that the liquid is itselfredox-active. Of course, such a liquid form may also be diluted by ormixed with another, non-redox-active liquid that is a diluent orsolvent, including mixing with such a diluent to form a lower-meltingliquid phase, emulsion or micelles including the ion-storing liquid.

In one aspect, a redox flow energy storage device is described. Theredox flow energy storage device includes:

-   -   a positive electrode current collector, a negative electrode        current collector, and an ion-permeable membrane separating the        positive and negative current collectors;    -   a positive electrode disposed between the positive electrode        current collector and the ion-permeable membrane; the positive        electrode current collector and the ion-permeable membrane        defining a positive electroactive zone accommodating the        positive electrode;    -   a negative electrode disposed between the negative electrode        current collector and the ion-permeable membrane; the negative        electrode current collector and the ion-permeable membrane        defining a negative electroactive zone accommodating the        negative electrode;    -   where at least one of the positive and negative electrode        includes a flowable semi-solid or condensed liquid ion-storing        redox composition which is capable of taking up or releasing the        ions during operation of the cell.

In some embodiments, both of the positive and negative electrodes of theredox flow energy storage device include the flowable semi-solid orcondensed liquid ion-storing redox compositions.

In some embodiments, one of the positive and negative electrodes of theredox flow energy storage device includes the flowable semi-solid orcondensed liquid ion-storing redox composition, and the remainingelectrode is a conventional stationary electrode.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition includes a gel.

In some embodiments, the steady state shear viscosity of the flowablesemi-solid or condensed liquid ion-storing redox composition of theredox flow energy storage device is between about 1 cP and about1,500,000 cP or between about 1 cP and 1,000,000 cP at the temperatureof operation of the redox flow energy storage device.

In some embodiments, the ion is selected from the group consisting ofLi⁺, Na⁺, H⁺.

In some embodiments, the ion is selected from the group consisting ofLi⁺ and Na⁺, Mg²⁺, Al³⁺, and Ca²⁺.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including an ion storage compound.

In some embodiments, the ion is a proton or hydroxyl ion and the ionstorage compound includes those used in a nickel-cadmium or nickel metalhydride battery.

In some embodiments, the ion is lithium and the ion storage compound isselected from the group consisting of metal fluorides such as CuF₂,FeF₂, FeF₃, BiF₃, CoF₂, and NiF₂.

In some embodiments, the ion is lithium and the ion storage compound isselected from the group consisting of metal oxides such as CoO, Co₃O₄,NiO, CuO, and MnO.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula Li_(1-x-z)M_(1-z)PO₄, wherein M includes at least one first rowtransition metal selected from the group consisting of Ti, V, Cr, Mn,Fe, Co and Ni, wherein x is from 0 to 1 and z can be positive ornegative.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula (Li_(1-x)Z_(x))MPO₄, where M is one or more of V, Cr, Mn, Fe,Co, and Ni, and Z is a non-alkali metal dopant such as one or more ofTi, Zr, Nb, Al, or Mg, and x ranges from 0.005 to 0.05.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from compounds with theformula LiMPO₄, where M is one or more of V, Cr, Mn, Fe, Co, and Ni, inwhich the compound is optionally doped at the Li, M or O-sites.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting ofA_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z), A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)z,and A_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), wherein x, plus y(1−a) times aformal valence or valences of M′, plus ya times a formal valence orvalence of M″, is equal to z times a formal valence of the XD₄, X₂D₇, orDXD₄ group; and A is at least one of an alkali metal and hydrogen, M′ isa first-row transition metal, X is at least one of phosphorus, sulfur,arsenic, molybdenum, and tungsten, M″ is any of a Group IIA, IIIA, IVA,VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, and D is atleast one of oxygen, nitrogen, carbon, or a halogen.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting of(A_(1-a)M″_(a))_(x)M′_(y)(XD₄)_(z), (A_(1-a)M″_(a))_(x)M′_(y)(DXD₄)z and(A_(1-a)M′)_(x)M′_(y)(X₂D₇)_(z), where (1−a)x plus the quantity ax timesthe formal valence or valences of M″ plus y times the formal valence orvalences of M′ is equal to z times the formal valence of the XD₄, X₂D₇or DXD₄ group, and A is at least one of an alkali metal and hydrogen, M′is a first-row transition metal, X is at least one of phosphorus,sulfur, arsenic, molybdenum, and tungsten, M″ any of a Group IIA, IIIA,IVA, VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, and Dis at least one of oxygen, nitrogen, carbon, or a halogen.

In some embodiments, the ion is lithium and the ion storage compoundincludes an intercalation compound selected from the group consisting ofordered rocksalt compounds LiMO₂ including those having the α-NaFeO₂ andorthorhombic-LiMnO₂ structure type or their derivatives of differentcrystal symmetry, atomic ordering, or partial substitution for themetals or oxygen, where M includes at least one first-row transitionmetal but may include non-transition metals including but not limited toAl, Ca, Mg, or Zr.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including amorphous carbon, disorderedcarbon, graphitic carbon, or a metal-coated or metal-decorated carbon.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including a metal or metal alloy ormetalloid or metalloid alloy or silicon.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including nanostructures includingnanowires, nanorods, nanotripods, and nanotetrapods.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a solid including an organic redox compound.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a solid selected fromthe group consisting of ordered rocksalt compounds LiMO₂ including thosehaving the α-NaFeO₂ and orthorhombic-LiMnO₂ structure type or theirderivatives of different crystal symmetry, atomic ordering, or partialsubstitution for the metals or oxygen, wherein M includes at least onefirst-row transition metal but may include non-transition metalsincluding but not limited to Al, Ca, Mg, or Zr and the negativeelectrode includes a flowable semi-solid ion-storing redox compositionincluding a solid selected from the group consisting of amorphouscarbon, disordered carbon, graphitic carbon, or a metal-coated ormetal-decorated carbon.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a solid selected fromthe group consisting of A_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z),A_(x)(M′_(1-a)M″_(a))_(y)(DXD₄)_(z), andA_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z), and where x, plus y(1−a) times aformal valence or valences of M′, plus ya times a formal valence orvalence of M″, is equal to z times a formal valence of the XD₄, X₂D₇, orDXD₄ group, and A is at least one of an alkali metal and hydrogen, M′ isa first-row transition metal, X is at least one of phosphorus, sulfur,arsenic, molybdenum, and tungsten, M″ any of a Group IIA, IIIA, IVA, VA,VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIB metal, D is at leastone of oxygen, nitrogen, carbon, or a halogen and the negative electrodeincludes a flowable semi-solid ion-storing redox composition including asolid selected from the group consisting of amorphous carbon, disorderedcarbon, graphitic carbon, or a metal-coated or metal-decorated carbon.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a compound with aspinel structure.

In some embodiments, the positive electrode includes a flowablesemi-solid ion-storing redox composition including a compound selectedfrom the group consisting of LiMn₂O₄ and its derivatives; layered-spinelnanocomposites in which the structure includes nanoscopic regions havingordered rocksalt and spinel ordering; so-called “high voltage spinels”with a potential vs. Li/Li⁺ that exceeds 4.3V including but not limitedto LiNi_(0.5)Mn_(1.5)O₄; olivines LiMPO₄ and their derivatives, in whichM includes one or more of Mn, Fe, Co, or Ni, partially fluorinatedcompounds such as LiVPO₄F, other “polyanion” compounds as describedbelow, and vanadium oxides V_(x)O_(y) including V₂O₅ and V₆O₁₁.

In some embodiments, the negative electrode includes a flowablesemi-solid ion-storing redox composition including graphite, graphiticboron-carbon alloys, hard or disordered carbon, lithium titanate spinel,or a solid metal or metal alloy or metalloid or metalloid alloy thatreacts with lithium to form intermetallic compounds, including themetals Sn, Bi, Zn, Ag, and Al, and the metalloids Si and Ge.

In some embodiments, in order to increase the particle packing densityand therefore the energy density of the semi-solid suspension, whilestill maintaining a flowable semi-solid, the ion storage compoundparticles have a polydisperse size distribution in which the finestparticles present in at least 5 vol % of the total volume, is at least afactor of 5 smaller than the largest particles present in at least 5 vol% of the total volume.

In some embodiments, in order to increase the particle packing densityand therefore the energy density of the semi-solid suspension, whilestill maintaining a flowable semi-solid, the ion storage compoundparticles have a bidisperse size distribution (i.e., with two maxima inthe distribution of particle number versus particle size) in which thetwo maxima differ in size by at least a factor of 5.

In some embodiments, the sized distribution of ion storage compoundparticles in the semi-solid is polydisperse, and the particle packingfraction is at least 50 vol %, preferably at least 55 vol %, morepreferably at least 60 vol %, still more preferably at least 65 vol %,and still more preferably at least 70 vol %.

In some embodiments, the particles have morphology that is at leastequiaxed, and preferably spherical, in order to increase the flowabilityand decrease the viscosity of the semi-solid suspension whilesimultaneously achieving high particle packing density. In someembodiments the spherical particles are dense, and in other embodimentsthe spherical particles are porous. In some embodiments, the sphericalparticles are made by spray-drying a particle suspension to obtainspherical agglomerates of smaller particles.

In some embodiments, the particles of ion storage material used in thesemi-solid suspension are sufficiently large that surface forces do notprohibit them from achieving high tap density while dry, and highpacking density when formulated into a semi-solid suspension. In someembodiments, the particle size is at least 1 micrometer and preferablyat least 10 micrometers.

In some embodiments, high particle packing density is achievedsimultaneously with flowability and low viscosity by using dispersantsand surfactants well-known to those skilled in the arts of ceramicsprocessing and colloid chemistry. These additives may be, for example,organic molecules having a C₆ to C₁₂ backbone used to provide stericforces when adsorbed on the particles. Examples of such additivesinclude stearic acid, and the commercially available surfactantTriton-X-100.

In some embodiments, a redox mediator is used to improve charge transferwithin the semi-solid suspension. In some embodiments the redox mediatoris based on Fe²⁺ or V²⁺, V³⁺, or V⁴⁺. In one embodiment the redoxmediator is ferrocene.

In one embodiment, the flow battery uses dissolved redox ions as in aconventional aqueous or nonaquous flow battery, but the anolyte and/orcatholyte has an increased solubility for such ions by using as thesolvent an ionic liquid. In some embodiments, the redox chemistry isFe—Cr, vanadium redox, or a zinc-halogen chemistry.

In some embodiments, the redox flow energy storage device furtherincludes a storage tank for storing the flowable semi-solid or condensedliquid ion-storing redox composition, and the storage tank is in flowcommunication with the redox flow energy storage device.

In some embodiments, the redox flow energy storage device includes aninlet for introduction of the flowable semi-solid or condensed liquidion-storing redox composition into the positive/negative electroactivezone and an outlet for the exit of the flowable semi-solid or condensedliquid ion-storing redox composition out of the positive/negativeelectroactive zone. In some specific embodiments, the redox flow energystorage device further includes a fluid transport device to enable theflow communication. In certain specific embodiments, the fluid transportdevice is a pump. In certain specific embodiments, the pump is aperistaltic pump.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition further includes one or more additives. Incertain specific embodiments, the additives include a conductiveadditive. In certain other embodiments, the additive includes athickener. In yet other specific embodiments, the additive includes acompound that getters water.

In some embodiments, the flowable semi-solid ion-storing redoxcomposition includes a ion-storing solid coated with a conductivecoating material. In certain specific embodiments, the conductivecoating material has higher electron conductivity than the solid. Incertain specific embodiments, the solid is graphite and the conductivecoating material is a metal, metal carbide, metal nitride, or carbon. Incertain specific embodiments, the metal is copper.

In some embodiments, the redox flow energy storage device furtherincludes one or more reference electrodes.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox composition of the redox flow energy storage deviceprovides a specific energy of more than about 150 Wh/kg at a totalenergy of less than about 50 kWh.

In some embodiments, the semi-solid or condensed-liquid ion-storingmaterial of the redox flow energy storage device provides a specificenergy of more than about 200 Wh/kg at total energy less than about 100kWh, or more than about 250 Wh/kg at total energy less than about 300kWh.

In some embodiments, the condensed-liquid ion-storing material includesa liquid metal alloy.

In some embodiments, the ion-permeable membrane includespolyethyleneoxide (PEO) polymer sheets or Nafion™ membranes.

In some embodiments, a method of operating a redox flow energy storagedevice is described. The method includes:

providing a redox flow energy storage device including:

-   -   a positive electrode current collector, a negative electrode        current collector, and an ion-permeable membrane separating the        positive and negative current collectors;    -   a positive electrode disposed between the positive electrode        current collector and the ion-permeable membrane; the positive        electrode current collector and the ion-permeable membrane        defining a positive electroactive zone accommodating the        positive electrode;    -   a negative electrode disposed between the negative electrode        current collector and the ion-permeable membrane; the negative        electrode current collector and the ion-permeable membrane        defining a negative electroactive zone accommodating the        negative electrode;    -   where at least one of the positive and negative electrode        includes a flowable semi-solid or condensed liquid ion-storing        redox composition which is capable of taking up or releasing the        ions during operation of the cell;

transporting the flowable semi-solid or condensed liquid ion-storingredox composition into the electroactive zone during operation of thedevice.

In some embodiments, in the method of operating a redox flow energystorage device, at least a portion of the flowable semi-solid orcondensed liquid ion-storing redox composition in the electroactive zoneis replenished by introducing new semi-solid or condensed liquidion-storing redox composition into the electroactive zone duringoperation.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

transporting depleted semi-solid or condensed liquid ion-storingmaterial to a discharged composition storage receptacle for recycling orrecharging.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

applying an opposing voltage difference to the flowable redox energystorage device; and transporting charged semi-solid or condensed liquidion-storing redox composition out of the electroactive zone to a chargedcomposition storage receptacle during charging.

In some embodiments, the method of operating a redox flow energy storagedevice further includes:

applying an opposing voltage difference to the flowable redox energystorage device; and

transporting discharged semi-solid or condensed liquid ion-storing redoxcomposition into the electroactive zone to be charged.

In some embodiments, a method of use in which a rechargeable battery isprovided with a zero self-discharge rate is provided. The semi-solidflow batteries of the invention are constructed to permit valving off ofthe cathode and anode slurries permitting long “standby,” and then“restarted” by activating flow. For example, this mode of operationprovides the first rechargeable nickel metal hydride or lithium ionbatteries with zero self-discharge, analogous to primary thermalbatteries. Long standby without self discharge is desirable for manyapplications including auxiliary grid-connected or autonomous powersources, or hybrid and all-electric vehicles batteries where a vehiclemay sit unused for a long period of time. Optionally, the method of usemay include activating the semi-solid catholyte or anolyte prior torestarting the battery by stirring, mixing, agitation, ultrasonication,or heating.

As used herein, positive electrode and cathode are used interchangeably.As used herein, negative electrode and anode are used interchangeably.

The energy storage systems described herein can provide a high enoughspecific energy to permit, for example, extended driving range for anelectric vehicle, or provide a substantial improvement in specificenergy or energy density over conventional redox batteries forstationary energy storage, including for example applications in gridservices or storage of intermittent renewable energy sources such aswind and solar power.

In another aspect, a flow cell energy storage system includes a positiveelectrode current collector, a negative electrode current collector, andan ion-permeable membrane separating said positive and negative currentcollectors, positioned and arranged to define a positive electroactivezone and a negative electroactive zone; wherein at least one of saidpositive and negative electroactive zone comprises a flowable semi-solidcomposition comprising ion storage compound particles capable of takingup or releasing said ions during operation of the cell, and wherein theion storage compound particles have a polydisperse size distribution inwhich the finest particles present in at least 5 vol % of the totalvolume, is at least a factor of 5 smaller than the largest particlespresent in at least 5 vol % of the total volume.

In one or more embodiments, the finest particles present in at least 5vol % of the total volume, is at least a factor of 7 smaller than thelargest particles present in at least 5 vol % of the total volume, orthe finest particles present in at least 5 vol % of the total volume, isat least a factor of 10 smaller than the largest particles present in atleast 5 vol % of the total volume.

In one or more embodiments, the ion storage compound particles have abidisperse size distribution in which the two maxima differ in size byat least a factor of 5.

In one or more embodiments, the particle packing fraction is at least 50vol %, preferably at least 55 vol %, more preferably at least 60 vol %,still more preferably at least 65 vol %, and still more preferably atleast 70 vol %.

In one or more embodiments, the particles have morphology that is atleast equiaxed.

In one or more embodiments, the particle size of the maxima for thelarger particles is at least 1 micrometer and preferably at least 10micrometers.

In one or more embodiments, the system further includes a redoxmediator.

In one or more embodiments, the redox mediator is soluble in thesemi-solid composition and comprises multiple oxidation states.

In one or more embodiments, the redox mediator is comprises a redoxmetal ion selected from iron, vanadium, chromium and zinc and mixturesthereof.

In one or more embodiments, the redox mediator comprises ferrocene.

In one or more embodiments, the semi-solid ion-storing redox compositionfurther comprises an electrically conductive additive.

In one or more embodiments, the electronically conductive materialcomprises a conductive inorganic compound.

In one or more embodiments, the electronically conductive material isselected from the group consisting of metals, metal carbides, metalnitrides, metal oxides, and allotropes of carbon including carbon black,graphitic carbon, carbon fibers, carbon microfibers, vapor-grown carbonfibers (VGCF), fullerenic carbons including “buckyballs”, carbonnanotubes (CNTs), multiwall carbon nanotubes (MWNTs), single wall carbonnanotubes (SWNTs), graphene sheets or aggregates of graphene sheets, andmaterials comprising fullerenic fragments and mixtures thereof.

In one or more embodiments, the electronically conductive materialcomprises an electronically conductive polymer.

In one or more embodiments, the electronically conductive material isselected from the group consisting of polyaniline or polyacetylene basedconductive polymers or poly(3,4-ethylenedioxythiophene) (PEDOT),polypyrrole, polythiophene, poly(p-phenylene), poly(triphenylene),polyazulene, polyfluorene, polynaphtalene, polyanthracene, polyfuran,polycarbazole, tetrathiafulvalene-substituted polystyrene,ferrocence-substituted polyethylene, carbazole-substituted polyethylene,polyoxyphenazine, polyacenes, or poly(heteroacenes) and mixturesthereof.

In one or more embodiments, the additive coats the ion storage compoundparticles

In one or more embodiments, the one or both of the positive and negativecurrent collector is coated with an electronically conductive material.

In one or more embodiments, the conductive-coating material is selectedfrom the group consisting of carbon, a metal, metal carbide, metalnitride, metal oxide, or conductive polymer, conductive polymers,polyaniline or polyacetylene based conductive polymers orpoly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polythiophene,poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene,polynaphtalene, polyanthracene, polyfuran, polycarbazole,tetrathiafulvalene-substituted polystyrene, ferrocence-substitutedpolyethylene, carbazole-substituted polyethylene, polyoxyphenazine,polyacenes, or poly(heteroacenes) and mixtures thereof.

In one or more embodiments, the conductive polymer is a compound thatreacts in-situ to form a conductive polymer on the surface of thecurrent collector.

In one or more embodiments, the compound comprises 2-hexylthiophene andoxidizes at a high potential to form a conductive polymer coating on thecurrent collector.

In another aspect, a flow cell energy storage system includes a positiveelectrode current collector, a negative electrode current collector, andan ion-permeable membrane separating said positive and negative currentcollectors, positioned and arranged to define a positive electroactivezone and a negative electroactive zone; wherein at least one of saidpositive and negative electroactive zone comprises a flowable condensedliquid composition comprising ion storage compound capable of taking upor releasing said ions during operation of the cell and anelectronically conductive polymer.

In one or more embodiments, the electronically conductive material isselected from the group consisting of polyaniline or polyacetylene basedconductive polymers or poly(3,4-ethylenedioxythiophene) (PEDOT),polypyrrole, polythiophene, poly(p-phenylene), poly(triphenylene),polyazulene, polyfluorene, polynaphtalene, polyanthracene, polyfuran,polycarbazole, tetrathiafulvalene-substituted polystyrene,ferrocence-substituted polyethylene, carbazole-substituted polyethylene,polyoxyphenazine, polyacenes, or poly(heteroacenes) and mixturesthereof.

In one aspect, a flow cell energy storage system includes a positiveelectrode current collector, a negative electrode current collector, andan ion-permeable membrane separating said positive and negative currentcollectors, positioned and arranged to define a positive electroactivezone and a negative electroactive zone; wherein at least one of saidpositive and negative electroactive zone comprises a flowable semi-solidor condensed liquid composition comprising ion storage compound capableof taking up or releasing said ions during operation of the cell, atleast one storage tank external to the flow cell for holding, deliveringand/or receiving the flowable semi-solid or condensed liquidcomposition; and a cut-off valve for reversibly isolating the storagetank from the flow cell.

In one aspect, the flow cell energy storage system a positive electrodecurrent collector, a negative electrode current collector, and anion-permeable membrane separating said positive and negative currentcollectors, positioned and arranged to define a positive electroactivezone and a negative electroactive zone; wherein at least one of saidpositive and negative electroactive zone comprises an aqueous redoxsolution capable of taking up or releasing said ions during operation ofthe cell and an electronically conductive additive.

In one or more embodiments, the electronically conductive material isselected from the group consisting of polyaniline or polyacetylene basedconductive polymers or poly(3,4-ethylenedioxythiophene) (PEDOT),polypyrrole, polythiophene, poly(p-phenylene), poly(triphenylene),polyazulene, polyfluorene, polynaphtalene, polyanthracene, polyfuran,polycarbazole, tetrathiafulvalene-substituted polystyrene,ferrocence-substituted polyethylene, carbazole-substituted polyethylene,polyoxyphenazine, polyacenes, or poly(heteroacenes) and mixturesthereof.

In one or more embodiments, the electronically conductive material isselected from the group consisting of solid inorganic conductivematerials, metals, metal carbides, metal nitrides, metal oxides, andallotropes of carbon including carbon black, graphitic carbon, carbonfibers, carbon microfibers, vapor-grown carbon fibers (VGCF), fullereniccarbons including “buckyballs”, carbon nanotubes (CNTs), multiwallcarbon nanotubes (MWNTs), single wall carbon nanotubes (SWNTs), graphenesheets or aggregates of graphene sheets, and materials comprisingfullerenic fragments and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter is described with reference to the drawings, whichare intended to be illustrative in nature and not intended to belimiting of the invention, the full scope of which is set forth in theclaims that follow.

FIG. 1 is a cross-sectional illustration of the redox flow batteryaccording to one or more embodiments.

FIG. 2 is a schematic illustration of an exemplary redox flow cell for alithium battery system.

FIG. 3 is a schematic illustration of an exemplary redox flow cell for anickel battery system.

FIG. 4 is a schematic illustration of an exemplary redox flow batteryusing reference electrodes to monitor and optimize cell performance.

FIG. 5 illustrates cycling performance of anode slurries with varyingcopper plating load.

FIG. 6 illustrates a representative plot of voltage as a function ofcharging capacity for the cathode slurry half-cell.

FIG. 7 illustrates a representative plot of voltage as a function ofcharging capacity for the anode slurry half-cell.

FIG. 8 illustrates a representative plot of voltage as a function oftime (lower panel) and the corresponding charge or discharge capacity(upper panel) for a electrochemical cell with cathode and anodeslurries.

FIG. 9 illustrates a representative plot of the cathode dischargecapacity vs. cycle number.

FIG. 10 illustrates the galvanostatic lithium insertion and extractioncurves for the suspension at a relatively high C/1.4 rate.

DETAILED DESCRIPTION

An exemplary redox flow energy storage device 100 is illustrated in FIG.1A. Redox flow energy storage device 100 may include a positiveelectrode current collector 110 and a negative electrode currentcollector 120, separated by an ion permeable separator 130. Currentcollectors 110, 120 may be in the form of a thin sheet and are spacedapart from separator 130. Positive electrode current collector 110 andion permeable separator 130 define an area, 115, herein after referredto as the “positive electroactive zone” that accommodates the positiveflowable electrode active material 140. Negative electrode currentcollector 120 and ion permeable separator 130 define an area, 125,herein after referred to as the “negative electroactive zone” thataccommodates the negative flowable electrode active material 150. Theelectrode-active materials can be flowable redox compositions and can betransported to and from the electroactive zone at which theelectrochemical reaction occurs. The flowable redox composition caninclude a semi-solid or a condensed liquid ion-storing electroactivematerial, and optionally a fluid for supporting or suspending the solidor condensed ion-storing liquid electrolyte. As used herein, semi-solidrefers to a mixture of liquid and solid phases, such as a slurry,particle suspension, colloidal suspension, emulsion, or micelle. In someembodiments, the emulsion or micelle in a semi-solid includes a solid inat least one of the liquid-containing phases. As used herein, condensedliquid or condensed ion-storing liquid refers to a liquid that is notmerely a solvent as it is in the case of an aqueous flow cell catholyteor anolyte, but rather that the liquid is itself redox-active. Theliquid form can also be diluted by or mixed with another,non-redox-active liquid that is a diluent or solvent, including mixingwith such a diluents to form a lower-melting liquid phase, emulsion ormicelles including the ion-storing liquid.

The positive electrode flowable material 140 can enter the positiveelectroactive zone 115 in the direction indicated by arrow 160. Positiveelectrode material 140 can flow through the electroactive zone and exitat the upper location of the electroactive zone in the directionindicated by arrow 165. Similarly, the negative electrode flowablematerial 150 can enter the negative electroactive zone 125 in thedirection indicated by arrow 170. Negative electrode material 150 canflow through the electroactive zone and exits at the upper location ofthe electroactive zone in the direction indicated by arrow 175. Thedirection of flow can be reversed, for example, when alternating betweencharging and discharging operations. It is noted that the illustrationof the direction of flow is arbitrary in the figure. Flow can becontinuous or intermittent. In some embodiments, the positive andnegative redox flow materials are stored in a storage zone or tank (notshown) prior to use. In some embodiments, the flowable redox electrodematerials can be continuously renewed and replaced from the storagezones, thus generating an energy storage system with very high energycapacity. In some embodiments, a transporting device is used tointroduce positive and negative ion-storing electroactive materials intothe positive and negative electroactive zones, respectively. In someembodiments, a transporting device is used to transport depletedpositive and negative ion-storing electroactive materials out of thepositive and negative electroactive zones, respectively, and intostorage tanks for depleted electroactive materials for recharging. Insome embodiments, the transporting device can be a pump or any otherconventional device for fluid transport. In some specific embodiments,the transporting device is a peristaltic pump.

During operation, the positive and negative electroactive materials canundergo reduction and oxidation. Ions 190 can move across ion permeablemembrane 130 and electrons can flow through an external circuit 180 togenerate current. In a typical flow battery, the redox-active ions orion complexes undergo oxidation or reduction when they are in closeproximity to or in contact with a current collector that typically doesnot itself undergo redox activity. Such a current collector may be madeof carbon or nonreactive metal, for example. Thus, the reaction rate ofthe redox active species can be determined by the rate with which thespecies are brought close enough to the current collector to be inelectrical communication, as well as the rate of the redox reaction onceit is in electrical communication with the current collector. In someinstances, the transport of ions across the ionically conductingmembrane may rate-limit the cell reaction. Thus the rate of charge ordischarge of the flow battery, or the power to energy ratio, may berelatively low. The number of battery cells or total area of theseparators or electroactive zones and composition and flow rates of theflowable redox compositions can be varied to provide sufficient powerfor any given application.

In some embodiments, at least one of the positive or negative flowableredox compositions includes a semi-solid or a condensed ion-storingliquid electroactive material.

During discharging operation, the difference in electrochemicalpotentials of the positive and negative electrode of the redox flowdevice can produces a voltage difference between the positive andnegative electrodes; this voltage difference would produce an electriccurrent if the electrodes were connected in a conductive circuit. Insome embodiments, during discharging, a new volume of charged flowablesemi-solid or condensed liquid ion-storing composition is transportedfrom a charged composition storage tank into the electroactive zone. Insome embodiments, during discharging, the discharged or depletedflowable semi-solid or condensed liquid ion-storing composition can betransported out of the electroactive zone and stored in a dischargedcomposition storage receptacle until the end of the discharge.

During charging operation, the electrode containing flowable redoxcomposition can be run in reverse, either electrochemically andmechanically. In some embodiments, the depleted flowable semi-solid orcondensed liquid ion-storing composition can be replenished bytransporting the depleted redox composition out of the electroactivezone and introducing fully charged flowable semi-solid or condensedliquid ion-storing composition into the electroactive zone. This couldbe accomplished by using a fluid transportation device such as a pump.In some other embodiments, an opposing voltage difference can be appliedto the flowable redox energy storage device to drive electronic currentand ionic current in a direction opposite to that of discharging, toreverse the electrochemical reaction of discharging, thus charging theflowable redox composition of the positive and negative electrodes. Insome specific embodiments, during charging, discharged or depletedflowable semi-solid or condensed liquid ion-storing composition ismechanically transported into the electroactive zone to be charged underthe opposing voltage difference applied to the electrodes. In somespecific embodiments, the charged flowable semi-solid or condensedliquid ion-storing composition is transported out of the electroactivezone and stored in a charged composition storage receptacle until theend of the charge. The transportation can be accomplished by using afluid transportation device such as a pump.

One distinction between a conventional flow battery anolyte andcatholyte and the ion-storing solid or liquid phases as exemplifiedherein is the molar concentration or molarity of redox species in thestorage compound. For example, conventional anolytes or catholytes thathave redox species dissolved in aqueous solution may be limited inmolarity to typically 2M to 8M concentration. Highly acidic solutionsmay be necessary to reach the higher end of this concentration range. Bycontrast, any flowable semi-solid or condensed liquid ion-storing redoxcomposition as described herein may have, when taken in moles per literor molarity, at least 10M concentration of redox species, preferably atleast 12M, still preferably at least 15M, and still preferably at least20M. The electrochemically active material can be an ion storagematerial or any other compound or ion complex that is capable ofundergoing Faradaic reaction in order to store energy. The electroactivematerial can also be a multiphase material including the above-describedredox-active solid or liquid phase mixed with a non-redox-active phase,including solid-liquid suspensions, or liquid-liquid multiphasemixtures, including micelles or emulsions having a liquid ion-storagematerial intimately mixed with a supporting liquid phase. In the case ofboth semi-solid and condensed liquid storage compounds for the flowableion-storing redox compositions, systems that utilize various workingions are contemplated, including aqueous systems in which H⁺ or OH⁻ arethe working ions, nonaqueous systems in which Li₊, Na⁺, or other alkaliions are the working ions, even alkaline earth working ions such as Ca²⁺and Mg²⁺, or Al³⁺. In each of these instances, a negative electrodestorage material and a positive electrode storage material may berequired, the negative electrode storing the working ion of interest ata lower absolute electrical potential than the positive electrode. Thecell voltage can be determined approximately by the difference inion-storage potentials of the two ion-storage electrode materials.

Systems employing both negative and positive ion-storage materials areparticularly advantageous because there are no additionalelectrochemical byproducts in the cell. Both the positive and negativeelectrodes materials are insoluble in the flow electrolyte and theelectrolyte does not become contaminated with electrochemicalcomposition products that must be removed and regenerated. In addition,systems employing both negative and positive lithium ion-storagematerials are particularly advantageous when using non-aqueouselectrochemical compositions.

In some embodiments, the flowable semi-solid or condensed liquidion-storing redox compositions include materials proven to work inconventional, solid lithium-ion batteries. In some embodiments, thepositive flowable electroactive materials contains lithium positiveelectroactive materials and the lithium cations are shuttled between thenegative electrode and positive electrode, intercalating into solid,host particles suspended in a liquid electrolyte.

In some embodiments at least one of the energy storage electrodesincludes a condensed ion-storing liquid of a redox-active compound,which may be organic or inorganic, and includes but is not limited tolithium metal, sodium metal, lithium-metal alloys, gallium and indiumalloys with or without dissolved lithium, molten transition metalchlorides, thionyl chloride, and the like, or redox polymers andorganics that are liquid under the operating conditions of the battery.Such a liquid form may also be diluted by or mixed with another,non-redox-active liquid that is a diluent or solvent, including mixingwith such a diluents to form a lower-melting liquid phase. However,unlike a conventional flow cell catholyte or anolyte, the redox activecomponent will comprise by mass at least 10% of the total mass of theflowable electrolyte, and preferably at least 25%.

In some embodiments, the redox-active electrode material, whether usedas a semi-solid or a condensed liquid format as defined above, comprisesan organic redox compound that stores the working ion of interest at apotential useful for either the positive or negative electrode of abattery. Such organic redox-active storage materials include “p”-dopedconductive polymers such as polyaniline or polyacetylene basedmaterials, polynitroxide or organic radical electrodes (such as thosedescribed in: H. Nishide et al., Electrochim. Acta, 50, 827-831, (2004),and K. Nakahara, et al., Chem. Phys. Lett., 359, 351-354 (2002)),carbonyl based organics, and oxocarbons and carboxylate, includingcompounds such as Li₂C₆O₆, Li₂C₈H₄O₄, and Li₂C₆H₄O₄ (see for example M.Armand et al., Nature Materials, DOI: 10.1038/nmat2372) and organosulfurcompounds.

In some embodiments, organic redox compounds that are electronicallyinsulating are used. In some instance, the redox compounds are in acondensed liquid phase such as liquid or flowable polymers that areelectronically insulating. In such cases, the redox active slurry may ormay not contain an additional carrier liquid. Additives can be combinedwith the condensed phase liquid redox compound to increase electronicconductivity. In some embodiments, such electronically insulatingorganic redox compounds are rendered electrochemically active by mixingor blending with particulates of an electronically conductive material,such as solid inorganic conductive materials including but not limitedto metals, metal carbides, metal nitrides, metal oxides, and allotropesof carbon including carbon black, graphitic carbon, carbon fibers,carbon microfibers, vapor-grown carbon fibers (VGCF), fullerenic carbonsincluding “buckyballs”, carbon nanotubes (CNTs), multiwall carbonnanotubes (MWNTs), single wall carbon nanotubes (SWNTs), graphene sheetsor aggregates of graphene sheets, and materials comprising fullerenicfragments.

In some embodiments, such electronically insulating organic redoxcompounds are rendered electronically active by mixing or blending withan electronically conductive polymer, including but not limited topolyaniline or polyacetylene based conductive polymers orpoly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polythiophene,poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene,polynaphtalene, polyanthracene, polyfuran, polycarbazole,tetrathiafulvalene-substituted polystyrene, ferrocence-substitutedpolyethylene, carbazole-substituted polyethylene, polyoxyphenazine,polyacenes, or poly(heteroacenes. The conductive additives form anelectrically conducting framework within the insulating liquid redoxcompounds that significantly increases the electrically conductivity ofthe composition. In some embodiments, the conductive addition forms apercolative pathway to the current collector.

In some embodiments the redox-active electrode material comprises a solor gel, including for example metal oxide sols or gels produced by thehydrolysis of metal alkoxides, amongst other methods generally known as“sol-gel processing.” Vanadium oxide gels of composition VxOy areamongst such redox-active sol-gel materials.

Other suitable positive active materials include solid compounds knownto those skilled in the art as those used in NiMH (Nickel-Metal Hydride)Nickel Cadmium (NiCd) batteries. Still other positive electrodecompounds for Li storage include those used in carbon monofluoridebatteries, generally referred to as CFx, or metal fluoride compoundshaving approximate stoichiometry MF₂ or MF₃ where M comprises Fe, Bi,Ni, Co, Ti, V. Examples include those described in H. Li, P. Balaya, andJ. Maier, Li-Storage via Heterogeneous Reaction in Selected Binary MetalFluorides and Oxides, Journal of The Electrochemical Society, 151 [11]A1878-A1885 (2004), M. Bervas, A. N. Mansour, W.-S. Woon, J. F.Al-Sharab, F. Badway, F. Cosandey, L. C. Klein, and G. G. Amatucci,“Investigation of the Lithiation and Delithiation Conversion Mechanismsin a Bismuth Fluoride Nanocomposites”, J. Electrochem. Soc., 153, A799(2006), and I. Plitz, F. Badway, J. Al-Sharab, A. DuPasquier, F.Cosandey and G. G. Amatucci, “Structure and Electrochemistry ofCarbon-Metal Fluoride Nanocomposites Fabricated by a Solid State RedoxConversion Reaction”, J. Electrochem. Soc., 152, A307 (2005).

As another example, fullerenic carbon including single-wall carbonnanotubes (SWNTs), multiwall carbon nanotubes (MWNTs), or metal ormetalloid nanowires may be used as ion-storage materials. One example isthe silicon nanowires used as a high energy density storage material ina report by C. K. Chan, H. Peng, G. Liu, K. Mcllwrath, X. F. Zhang, R.A. Huggins, and Y. Cui, High-performance lithium battery anodes usingsilicon nanowires, Nature Nanotechnology, published online 16 Dec. 2007;doi:10.1038/nnano.2007.411.

Exemplary electroactive materials for the positive electrode in alithium system include the general family of ordered rocksalt compoundsLiMO₂ including those having the a-NaFeO₂ (so-called “layeredcompounds”) or orthorhombic-LiMnO₂ structure type or their derivativesof different crystal symmetry, atomic ordering, or partial substitutionfor the metals or oxygen. M comprises at least one first-row transitionmetal but may include non-transition metals including but not limited toAl, Ca, Mg, or Zr. Examples of such compounds include LiCoO₂, LiCoO₂doped with Mg, LiNiO₂, Li(Ni, Co, Al)O₂ (known as “NCA”) and Li(Ni, Mn,Co)O₂ (known as “NMC”). Other families of exemplary electroactivematerials includes those of spinel structure, such as LiMn₂O₄ and itsderivatives, so-called “layered-spinel nanocomposites” in which thestructure includes nanoscopic regions having ordered rocksalt and spinelordering, olivines LiMPO₄ and their derivatives, in which M comprisesone or more of Mn, Fe, Co, or Ni, partially fluorinated compounds suchas LiVPO₄F, other “polyanion” compounds as described below, and vanadiumoxides V_(x)O_(y) including V₂O₅ and V₆O₁₁.

In one or more embodiments the active material comprises a transitionmetal polyanion compound, for example as described in U.S. Pat. No.7,338,734. In one or more embodiments the active material comprises analkali metal transition metal oxide or phosphate, and for example, thecompound has a composition A_(x)(M′_(1-a)M″_(a))_(y)(XD₄)_(z),A_(x)(M′_(1-a)M″)_(y)(DXD₄)_(z), or A_(x)(M′_(1-a)M″_(a))_(y)(X₂D₇)_(z),and have values such that x, plus y(1−a) times a formal valence orvalences of M′, plus ya times a formal valence or valence of M″, isequal to z times a formal valence of the XD₄, X₂D₇, or DXD₄ group; or acompound comprising a composition (A_(1-a)M″_(a))_(x)M′_(y)(XD₄)^(z),(A^(1-a)M″_(a))_(x)(M′_(y)(DXD₄)z(A_(1-a)M″)_(x)M′_(y)(X₂D₇)_(z) andhave values such that (1−a)x plus the quantity ax times the formalvalence or valences of M″ plus y times the formal valence or valences ofM′ is equal to z times the formal valence of the XD₄, X₂D₇ or DXD₄group. In the compound, A is at least one of an alkali metal andhydrogen, M′ is a first-row transition metal, X is at least one ofphosphorus, sulfur, arsenic, molybdenum, and tungsten, M″ any of a GroupIIA, IIIA, IVA, VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, VB, and VIBmetal, D is at least one of oxygen, nitrogen, carbon, or a halogen. Thepositive electroactive material can be an olivine structure compoundLiMPO₄, where M is one or more of V, Cr, Mn, Fe, Co, and Ni, in whichthe compound is optionally doped at the Li, M or O-sites. Deficienciesat the Li-site are compensated by the addition of a metal or metalloid,and deficiencies at the O-site are compensated by the addition of ahalogen. In some embodiments, the positive active material comprises athermally stable, transition-metal-doped lithium transition metalphosphate having the olivine structure and having the formula(Li_(1-x)Z_(x))MPO₄, where M is one or more of V, Cr, Mn, Fe, Co, andNi, and Z is a non-alkali metal dopant such as one or more of Ti, Zr,Nb, Al, or Mg, and x ranges from 0.005 to 0.05.

In other embodiments, the lithium transition metal phosphate materialhas an overall composition of Li_(1-x-z)M_(1+z)PO₄, where M comprises atleast one first row transition metal selected from the group consistingof Ti, V, Cr, Mn, Fe, Co and Ni, where x is from 0 to 1 and z can bepositive or negative. M includes Fe, z is between about 0.15 and −0.15.The material can exhibit a solid solution over a composition range of0<x<0.15, or the material can exhibit a stable solid solution over acomposition range of x between 0 and at least about 0.05, or thematerial can exhibit a stable solid solution over a composition range ofx between 0 and at least about 0.07 at room temperature (22-25° C.). Thematerial may also exhibit a solid solution in the lithium-poor regime,e.g., where x≧0.8, or x≧0.9, or x≧0.95.

In some embodiments the redox-active electrode material comprises ametal salt that stores an alkali ion by undergoing a displacement orconversion reaction. Examples of such compounds include metal oxidessuch as CoO, Co₃O₄, NiO, CuO, MnO, typically used as a negativeelectrode in a lithium battery, which upon reaction with Li undergo adisplacement or conversion reaction to form a mixture of Li₂O and themetal constituent in the form of a more reduced oxide or the metallicform. Other examples include metal fluorides such as CuF₂, FeF₂, FeF₃,BiF₃, CoF₂, and NiF₂, which undergo a displacement or conversionreaction to form LiF and the reduced metal constituent. Such fluoridesmay be used as the positive electrode in a lithium battery. In otherembodiments the redox-active electrode material comprises carbonmonofluoride or its derivatives. In some embodiments the materialundergoing displacement or conversion reaction is in the form ofparticulates having on average dimensions of 100 nanometers or less. Insome embodiments the material undergoing displacement or conversionreaction comprises a nanocomposite of the active material mixed with aninactive host, including but not limited to conductive and relativelyductile compounds such as carbon, or a metal, or a metal sulfide. FeS₂and FeF₃ can also be used as cheap and electronically conductive activematerials in a nonaqueous or aqueous lithium system.

In some embodiments the semi-solid flow battery is a lithium battery,and the negative electrode active compound comprises graphite, graphiticboron-carbon alloys, hard or disordered carbon, lithium titanate spinel,or a solid metal or metal alloy or metalloid or metalloid alloy thatreacts with lithium to form intermetallic compounds, including themetals Sn, Bi, Zn, Ag, and Al, and the metalloids Si and Ge.

Exemplary electroactive materials for the negative electrode in the caseof a lithium working ion include graphitic or non-graphitic carbon,amorphous carbon, or mesocarbon microbeads; an unlithiated metal ormetal alloy, such as metals including one or more of Ag, Al, Au, B, Ga,Ge, In, Sb, Sn, Si, or Zn, or a lithiated metal or metal alloy includingsuch compounds as LiAl, Li₉Al₄, Li₃Al, LiZn, LiAg, Li₁₀Ag₃, Li₅B₄,Li₇B₆, Li₁₂Si₇, Li₂₁Si₈, Li₁₃Si₄, Li₂₁Si₅, Li₅Sn₂, Li₁₃Sn₅, Li₇Sn₂,Li₂₂Sn₅, Li₂Sb, Li₃Sb, LiBi, or Li₃Bi, or amorphous metal alloys oflithiated or non-lithiated compositions.

The current collector can be electronically conductive and should beelectrochemically inactive under the operation conditions of the cell.Typical current collectors for lithium cells include copper, aluminum,or titanium for the negative current collector and aluminum for thepositive current collector, in the form of sheets or mesh, or anyconfiguration for which the current collector may be distributed in theelectrolyte and permit fluid flow. Selection of current collectormaterials is well-known to those skilled in the art. In someembodiments, aluminum is used as the current collector for positiveelectrode. In some embodiments, copper is used as the current collectorfor negative electrode. In other embodiments, aluminum is used as thecurrent collector for negative electrode.

In some embodiments, the negative electrode can be a conventionalstationary electrode, while the positive electrode includes a flowableredox composition. In other embodiments, the positive electrode can be aconventional stationary electrode, while the negative electrode includesa flowable redox composition.

Current collector materials can be selected to be stable at theoperating potentials of the positive and negative electrodes of the flowbattery. In nonaqueous lithium systems the positive current collectormay comprise aluminum, or aluminum coated with conductive material thatdoes not electrochemically dissolve at operating potentials of 2.5-5Vwith respect to Li/Li⁺. Such materials include Pt, Au, Ni, conductivemetal oxides such as vanadium oxide, and carbon. The negative currentcollector may comprise copper or other metals that do not form alloys orintermetallic compounds with lithium, carbon, and coatings comprisingsuch materials on another conductor.

In some embodiments the redox-active compound is present as a nanoscale,nanoparticle, or nanostructured form. This can facilitate the formationof stable liquid suspensions of the storage compound, and improves therate of reaction when such particles are in the vicinity of the currentcollector. The nanoparticulates may have equiaxed shapes or have aspectratios greater than about 3, including nanotubes, nanorods, nanowires,and nanoplatelets. Branched nanostructures such as nanotripods andnanotetrapods are also contemplated. Nanostructured ion storagecompounds may be prepared by a variety of methods including mechanicalgrinding, chemical precipitation, vapor phase reaction, laser-assistedreactions, and bio-assembly. Bio-assembly methods include, for example,using viruses having DNA programmed to template an ion-storing inorganiccompound of interest, as described in K. T. Nam, D. W. Kim, P. J. Yoo,C.-Y. Chiang, N. Meethong, P. T. Hammond, Y.-M. Chiang, A. M. Belcher,“Virus enabled synthesis and assembly of nanowires for lithium ionbattery electrodes,” Science, 312[5775], 885-888 (2006).

In redox cells with a semi-solid flowable redox composition, too fine asolid phase can inhibit the power and energy of the system by “clogging”the current collectors. In one or more embodiments, the semi-solidflowable composition contains very fine primary particle sizes for highredox rate, but which are aggregated into larger agglomerates. Thus insome embodiments, the particles of solid redox-active compound in thepositive or negative flowable redox compositions are present in a porousaggregate of 1 micrometer to 500 micrometer average diameter.

In some embodiments, in order to increase the particle packing densityand therefore the energy density of the semi-solid suspension, whilestill maintaining a flowable semi-solid, the ion storage compoundparticles have a polydisperse size distribution in which the finestparticles present in at least 5 vol % of the total volume, is at least afactor of 5 smaller than the largest particles present in at least 5 vol% of the total volume.

In some embodiments, in order to increase the particle packing densityand therefore the energy density of the semi-solid suspension, whilestill maintaining a flowable semi-solid, the ion storage compoundparticles have a bidisperse size distribution (i.e., with two maxima inthe distribution of particle number versus particle size) in which thetwo maxima differ in size by at least a factor of 5.

In some embodiments, the sized distribution of ion storage compoundparticles in the semi-solid is polydisperse, and the particle packingfraction is at least 50 vol %, preferably at least 55 vol %, morepreferably at least 60 vol %, still more preferably at least 65 vol %,and still more preferably at least 70 vol %. In one or more embodiments,the packing fraction is in the range of 50 vol % to 95 vol %.

In some embodiments, the particles have morphology that is at leastequiaxed, and preferably spherical, in order to increase the flowabilityand decrease the viscosity of the semi-solid suspension whilesimultaneously achieving high particle packing density. In someembodiments, the particles have an oblate spheroid particle shape. Insome embodiments the spherical particles are dense, and in otherembodiments the spherical particles are porous. In some embodiments, thespherical particles are made by spray-drying a particle suspension toobtain spherical agglomerates of smaller particles.

Particles with very small particle size, e.g., on the order of less than500 nm, can sometimes form low density continuous networks. Suchnetworks demonstrate shear thinning behavior and high viscosity at lowsolids content. The increased viscosity complicates fluid flow and thelow solids content reduces energy density. In some embodiments, theparticles of ion storage material used in the semi-solid suspension aresufficiently large that surface forces do not prohibit them fromachieving high tap density while dry, and high packing density whenformulated into a semi-solid suspension. In some embodiments, theparticle size is at least 1 micrometer and preferably at least 10micrometers. Particles in this size range provide adequate flowability,yet are coarse enough that gravity, not surface energy, is the dominantforce in particle packing.

Polydisperse size distribution of substantially equiaxed particles canprovide a high packing density while maintaining flowability of thesemisolid. Randomly packed monodisperse particles can become rigid atrelatively low packing densities, e.g. at a particle packing fraction ofabout 58 vol %. To provide a semi-solid composition of higher solidscontent, the solids content of a large particles, e.g., particles whosepacking is defined by gravity and not surface energy, is at a level atwhich the monodisperse particles are fluid or non-rigid. Additionalparticles of smaller particle size are introduced; such particles are ofa size that can fit into interstitial spaces arising from packing oflarger particles and are typically at least a factor of 5 smaller thanthe largest particles present. The smaller particles can occupy theinterstitial spaces and therefore cannot form low density continuousnetworks. Thus, a high solids content of the semi-solid composition isattained, without undesirably increasing viscosity and impairing flow.Exemplary particle packing fractions can be about 75-85%.

The ion-permeable medium through which ions are transported within theredox flow energy storage device can include any suitable medium capableof allowing ions to be passed through it. In some embodiments, theion-permeable medium can comprise a membrane. The membrane can be anyconventional membrane that is capable of ion transport. In one or moreembodiments, the membrane is a liquid-impermeable membrane that permitsthe transport of ions there through, namely a solid or gel ionicconductor. In other embodiments the membrane is a porous polymermembrane infused with a liquid electrolyte that allows for the shuttlingof ions between the anode and cathode electroactive materials, whilepreventing the transfer of electrons. In some embodiments, the membraneis a microporous membrane that prevents particles forming the positiveand negative electrode flowable compositions from crossing the membrane.Exemplary membrane materials include polyethyleneoxide (PEO) polymer inwhich a lithium salt is complexed to provide lithium conductivity, orNafion™ membranes which are proton conductors. For example, PEO basedelectrolytes can be used as the membrane, which is pinhole-free and asolid ionic conductor, optionally stabilized with other membranes suchas glass fiber separators as supporting layers. PEO can also be used asa slurry stabilizer, dispersant, etc. in the positive or negativeflowable redox compositions. PEO is stable in contact with typical alkylcarbonate-based electrolytes. This can be especially useful inphosphate-based cell chemistries with cell potential at the positiveelectrode that is less than about 3.6 V with respect to Li metal. Theoperating temperature of the redox cell can be elevated as necessary toimprove the ionic conductivity of the membrane.

In some embodiments, a carrier liquid is used to suspend and transportthe solid phase or condensed liquid of the flowable redox composition.The carrier liquid can be any liquid that can suspend and transport thesolid phase or condensed ion-storing liquid of the flowable redoxcomposition. By way of example, the carrier liquid can be water, a polarsolvent such as alcohols or aprotic organic solvents. Numerous organicsolvents have been proposed as the components of Li-ion batteryelectrolytes, notably a family of cyclic carbonate esters such asethylene carbonate, propylene carbonate, butylene carbonate, and theirchlorinated or fluorinated derivatives, and a family of acyclic dialkylcarbonate esters, such as dimethyl carbonate, diethyl carbonate,ethylmethyl carbonate, dipropyl carbonate, methyl propyl carbonate,ethyl propyl carbonate, dibutyl carbonate, butylmethyl carbonate,butylethyl carbonate and butylpropyl carbonate. Other solvents proposedas components of Li-ion battery electrolyte solutions includeγ-butyrolactone, dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether,sulfolane, methylsulfolane, acetonitrile, propiononitrile, ethylacetate, methyl propionate, ethyl propionate, dimethyl carbonate,tetraglyme, and the like. These nonaqueous solvents are typically usedas multicomponent mixtures, into which a salt is dissolved to provideionic conductivity. Exemplary salts to provide lithium conductivityinclude LiClO₄, LiPF₆, LiBF₄, LiTFSI, LiBETI, LiBOB, and the like.

In some embodiments, the viscosity of the redox compositions undergoingflow can be within a very broad range, from about 1 centipoise (cP) toabout 1.5×10⁶ cP or from about 1 centipoise (cP) to about 10⁶ cP at theoperating temperature of the battery, which may be between about −50° C.and +500° C. In some embodiments, the viscosity of the electrodeundergoing flow is less than about 10⁵ cP. In other embodiments, theviscosity is between about 100 cP and 10⁵ cP. In those embodiments wherea semi-solid is used, the volume percentage of ion-storing solid phasesmay be between 5% and 70%, and the total solids percentage includingother solid phases such as conductive additives may be between 10% and75%. In some embodiments, the cell “stack” where electrochemicalreaction occurs operates at a higher temperature to decrease viscosityor increase reaction rate, while the storage tanks for the semi-solidmay be at a lower temperature.

In some embodiments, peristaltic pumps are used to introduce asolid-containing electroactive material into an electroactive zone, ormultiple electroactive zones in parallel. The complete volume (occupiedby the tubing, a slurry reservoir, and the active cells) of the slurrycan be discharged and recharged by slurry cycling. The active positiveelectrode and negative electrode slurries can be independently cycledthrough the cell by means of peristaltic pumps. The pump can provideindependent control of the flow rates of the positive electrode slurryand the negative electrode slurry. The independent control permits powerbalance to be adjusted to slurry conductivity and capacity properties.

In some embodiments, the peristaltic pump works by moving a roller alonga length of flexible tubing. This way the fluid inside the tubing nevercomes into contact with anything outside of the tubing. In a pump, adrive turns a shaft which is coupled to a pump head. The pump headsecures the tubing in place and also use the rotation of the shaft tomove a rolling head across the tubing to create a flow within the tube.Such pumps are often used in situations where the fluid beingtransferred needs to be isolated (as in blood transfusions and othermedical applications). Here the peristaltic pump can also be used totransfer viscous fluids and particle suspensions. In some embodiments, aclosed circuit of tubing is used to run the slurry in a cycle, withpower provided by the peristaltic pump. In some embodiments, the closedanolyte and catholyte systems may be connected to removable reservoirsto collect or supply anolyte and catholyte; thus enabling the activematerial to be recycled externally. The pump will require a source ofpower which may include that obtained from the cell. In someembodiments, the tubing may not be a closed cycle, in which caseremovable reservoirs for charged and of discharged anolytes andcatholytes would be necessary; thus enabling the active material to berecycled externally. In some embodiments, one or more slurries arepumped through the redox cell at a rate permitting complete charge ordischarge during the residence time of the slurry in the cell, whereasin other embodiments one or more slurries are circulated repeatedlythrough the redox cell at a higher rate, and only partially charged ordischarged during the residence time in the cell. In some embodimentsthe pumping direction of one or more slurries is intermittently reversedto improve mixing of the slurries or to reduce clogging of passages inthe flow system.

While peristaltic pumps have been described in detail, it should beunderstood that other types of pumps can also be used to transport theflowable redox composition(s) described herein. For example, in someembodiments, a piston pump is used to transport one or more flowableredox compositions through the redox flow energy storage device.

The flowable redox compositions can include various additives to improvethe performance of the flowable redox cell. The liquid phase of thesemi-solid slurry in such instances would comprise a solvent, in whichis dissolved an electrolyte salt, and binders, thickeners, or otheradditives added to improve stability, reduce gas formation, improve SEIformation on the negative electrode particles, and the like. Examples ofsuch additives include vinylene carbonate (VC), vinylethylene carbonate(VEC), fluoroethylene carbonate (FEC), or alkyl cinnamates, to provide astable passivation layer on the anode or thin passivation layer on theoxide cathode; propane sultone (PS), propene sultone (PrS), or ethylenethiocarbonate as antigassing agents; biphenyl (BP), cyclohexylbenzene,or partially hydrogenated terphenyls, as gassing/safety/cathodepolymerization agents; or lithium bis(oxatlato)borate as an anodepassivation agent.

In some embodiments, the nonaqueous positive and negative electrodeflowable redox compositions are prevented from absorbing impurity waterand generating acid (such as HF in the case of LiPF₆ salt) byincorporating compounds that getter water into the active materialsuspension or into the storage tanks or other plumbing of the system.Optionally, the additives are basic oxides that neutralize the acid.Such compounds include but are not limited to silica gel, calciumsulfate (for example, the product known as Drierite), aluminum oxide andaluminum hydroxide.

In some embodiments, the colloid chemistry and rheology of thesemi-solid flow electrode is adjusted to produce a stable suspensionfrom which the solid particles settle only slowly or not at all, inorder to improve flowability of the semi-solid and to minimize anystirring or agitation needed to avoid settling of the active materialparticles. The stability of the electroactive material particlesuspension can be evaluated by monitoring a static slurry for evidenceof solid-liquid separation due to particle settling. As used herein, anelectroactive material particle suspension is referred to as “stable”when there is no observable particle settling in the suspension. In someembodiments, the electroactive material particle suspension is stablefor at least 5 days. Usually, the stability of the electroactivematerial particle suspension increases with decreased suspended particlesize. In some embodiments, the particle size of the electroactivematerial particle suspension is about less than 10 microns. In someembodiments, the particle size of the electroactive material particlesuspension is about less than 5 microns. In some embodiments, theparticle size of the electroactive material particle suspension is about2.5 microns. In some embodiments, conductive additives are added to theelectroactive material particle suspension to increase the conductivityof the suspension. Generally, higher volume fractions of conductiveadditives such as Ketjen carbon particles increase suspension stabilityand electronic conductivity, but excessive amount of conductiveadditives may also increase the viscosity of the suspension. In someembodiments, the flowable redox electrode composition includesthickeners or binders to reduce settling and improve suspensionstability. In some embodiments, the shear flow produced by the pumpsprovides additional stabilization of the suspension. In someembodiments, the flow rate is adjusted to eliminate the formation ofdendrites at the electrodes.

In some embodiments, the active material particles in the semi-solid areallowed to settle and are collected and stored separately, then re-mixedwith the liquid to form the flow electrode as needed.

In some embodiments, the rate of charge or discharge of the redox flowbattery is increased by increasing the instant amount of one or bothflow electrodes in electronic communication with the current collector.

In some embodiments, this is accomplished by making the semi-solidsuspension more electronically conductive, so that the reaction zone isincreased and extends into the flow electrode. In some embodiments, theconductivity of the semi-solid suspension is increased by the additionof a conductive material, including but not limited to metals, metalcarbides, metal nitrides, and forms of carbon including carbon black,graphitic carbon powder, carbon fibers, carbon microfibers, vapor-growncarbon fibers (VGCF), and fullerenes including “buckyballs”, carbonnanotubes (CNTs), multiwall carbon nanotubes (MWNTs), single wall carbonnanotubes (SWNTs), graphene sheets or aggregates of graphene sheets, andmaterials comprising fullerenic fragments that are not predominantly aclosed shell or tube of the graphene sheet. In some embodiments, nanorodor nanowire or highly expected particulates of active materials orconductive additives can be included in the electrode suspensions toimprove ion storage capacity or power or both. As an example, carbonnanofilters such as VGCF (vapor growth carbon fibers), multiwall carbonnanotubes (MWNTs) or single-walled carbon nanotubes (SWNTs), may be usedin the suspension to improve electronic conductivity, or optionally tostore the working ion.

In some embodiments, the electrochemical function of a conventionalaqueous or non-aqueous redox flow battery including those discussed inC. Ponce de Leon, A. Frias-Ferrer, J. Gonzalez-Garcia, D. A. Szantos andF. C. Walsh, “Redox Flow Batteries for Energy Conversion,” J. PowerSources, 160, 716 (2006), M. Bartolozzi, “Development of Redox FlowBatteries: A Historical Bibliography,” J. Power Sources, 27, 219 (1989),or M. Skyllas-Kazacos and F. Grossmith, “Efficient Vanadium Redox FlowCell,” Journal of the Electrochemical Society, 134, 2950 (1987), isimproved by mixing or blending the catholyte or anolyte withparticulates of an electronically conductive material, such as solidinorganic conductive materials including but not limited to metals,metal carbides, metal nitrides, metal oxides, and allotropes of carbonincluding carbon black, graphitic carbon, carbon fibers, carbonmicrofibers, vapor-grown carbon fibers (VGCF), fullerenic carbonsincluding “buckyballs”, carbon nanotubes (CNTs), multiwall carbonnanotubes (MWNTs), single wall carbon nanotubes (SWNTs), graphene sheetsor aggregates of graphene sheets, and materials comprising fullerenicfragments. In some embodiments, such electronically insulating organicredox compounds are rendered electronically active by mixing or blendingwith an electronically conductive polymer, including but not limited topolyaniline or polyacetylene based conductive polymers orpoly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polythiophene,poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene,polynaphtalene, polyanthracene, polyfuran, polycarbazole,tetrathiafulvalene-substituted polystyrene, ferrocence-substitutedpolyethylene, carbazole-substituted polyethylene, polyoxyphenazine,polyacenes, or poly(heteroacenes).). In some embodiments, the resultingcatholyte or anolyte mixture has an electronic conductivity of at least10⁻⁶ S/cm, preferably at least 10⁻⁵ S/cm, more preferably at least 10-4S/cm, and still more preferably at least 10-3 S/cm.

In some embodiments, the conductivity of the semi-solid ion-storingmaterial is increased by coating the solid of the semi-solid ion-storingmaterial with a conductive coating material which has higher electronconductivity than the solid. Non-limiting examples of conductive-coatingmaterial include carbon, a metal, metal carbide, metal nitride, metaloxide, or conductive polymer. In some embodiments, the coating can beconducted in a fluidized bed by electroplating of active particles witha metal; other techniques such as decorating active material with Cu (orother metal) through sintering is also contemplated.

In some embodiments, the conductive polymer includes but is not limitedto polyaniline or polyacetylene based conductive polymers orpoly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polythiophene,poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene,polynaphtalene, polyanthracene, polyfuran, polycarbazole,tetrathiafulvalene-substituted polystyrene, ferrocence-substitutedpolyethylene, carbazole-substituted polyethylene, polyoxyphenazine,polyacenes, or poly(heteroacenes).). In some embodiments, the conductivepolymer is a compound that reacts in-situ to form a conductive polymeron the surface of active materials particles. In one embodiment, thecompound is 2-hexylthiophene or 3-hexylthiophene and oxidizes duringcharging of the battery to form a conductive polymer coating on solidparticles in the cathode semi-solid suspension. In other embodiments,redox active material can be embedded in conductive matrix The redoxactive material can coat the exterior and interior interfaces in aflocculated or agglomerated particulate of conductive material. In otherembodiments, the redox-active material and the conductive material canbe two components of a composite particulate. Without being bound by anytheory or mode of operation, such coatings can passivate the redoxactive particles and can help prevent undesirable reactions with carrierliquid or electrolyte. As such, it can serve as a syntheticsolid-electrolyte interphase (SEI) layer.

In some embodiments, the solid of the semi-solid ion-storing material iscoated with metal that is redox-inert at the operating conditions of theredox energy storage device. In some embodiments, the solid of thesemi-solid ion-storing material is coated with copper to increase theconductivity of the storage material particle, to increase the netconductivity of the semi-solid, and/or to facilitate charge transferbetween energy storage particles and conductive additives. In someembodiments, the storage material particle is coated with, about 1.5% byweight, metallic copper. In some embodiments, the storage materialparticle is coated with, about 3.0% by weight, metallic copper. In someembodiments, the storage material particle is coated with, about 8.5% byweight, metallic copper. In some embodiments, the storage materialparticle is coated with, about 10.0% by weight, metallic copper. In someembodiments, the storage material particle is coated with, about 15.0%by weight, metallic copper. In some embodiments, the storage materialparticle is coated with, about 20.0% by weight, metallic copper.

In general, the cycling performance of the flowable redox electrodeincreases with the increases of the weight percentages of the conductivecoating material. In general, the capacity of the flowable redoxelectrode also increases with the increases of the weight percentages ofthe conductive coating material.

In some embodiments, the surface conductivity or charge-transferresistance of current collectors used in the semi-solid flow battery isincreased by coating the current collector surface with a conductivematerial. Such layers can also serve as a synthetic SEI layer.Non-limiting examples of conductive-coating material include carbon, ametal, metal carbide, metal nitride, metal oxide, or conductive polymer.In some embodiments, the conductive polymer includes but is not limitedto polyaniline or polyacetylene based conductive polymers orpoly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polythiophene,poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene,polynaphtalene, polyanthracene, polyfuran, polycarbazole,tetrathiafulvalene-substituted polystyrene, ferrocence-substitutedpolyethylene, carbazole-substituted polyethylene, polyoxyphenazine,polyacenes, or poly(heteroacenes). In some embodiments, the conductivepolymer is a compound that reacts in-situ to form a conductive polymeron the surface of the current collector. In one embodiment, the compoundis 2-hexylthiophene and oxidizes at a high potential to form aconductive polymer coating on the current collector. In someembodiments, the current collector is coated with metal that isredox-inert at the operating conditions of the redox energy storagedevice.

In some embodiments, a redox mediator is used to improve charge transferwithin the semi-solid suspension. The redox mediator assists in thetransfer of electrical current from the redox compound to the currentcollector. Redox mediators include soluble species having multipleoxidation states. In some embodiments the redox mediator is based onFe²⁺ or V²⁺, V³⁺, or V⁴⁺. In one embodiment the redox mediator isferrocene.

In one embodiment, the flow battery uses dissolved redox ions as in aconventional aqueous or nonaquous flow battery, but the anolyte and/orcatholyte has an increased solubility for such ions by using as thesolvent an ionic liquid. In some embodiments, the redox chemistry isFe—Cr, vanadium redox, or a zinc-halogen chemistry.

In some embodiments, the rate of charge or discharge of the redox flowbattery is increased by adjusting the interparticle interactions orcolloid chemistry of the semi-solid to increase particle contact and theformation of percolating networks of the ion-storage material particles.In some embodiments, the percolating networks are formed in the vicinityof the current collectors. In some embodiments, the semi-solid isshear-thinning so that it flows more easily where desired. In someembodiments, the semi-solid is shear thickening, for example so that itforms percolating networks at high shear rates such as those encounteredin the vicinity of the current collector.

The energy density of nonaqueous batteries using the flowable electrodeactive materials according to one or more embodiments compares favorablyto conventional redox anolyte and catholyte batteries. Redox anolytesand catholytes, for example those based on vanadium ions in solution,typically have a molar concentration of the vanadium ions of between 1and 8 molar, the higher concentrations occurring when high acidconcentrations are used. One may compare the energy density of asemi-solid slurry based on known lithium ion battery positive andnegative electrode compounds to these values. The liquid phase of thesemi-solid slurry in such instances would comprise a solvent, includingbut not limited to an alkyl carbonate or mixture of alkyl carbonates, inwhich is dissolved a lithium salt, including but not limited to LiPF₆,and binders, thickeners, or other additives added to improve stability,reduce gas formation, improve SEI formation on the negative electrodeparticles, and the like.

In a non-aqueous semi-solid redox flow cell, one useful positiveelectrode flowable redox composition is a suspension of lithiumtransition metal olivine particles in the liquid discussed above. Sucholivines include LiMPO₄ where M comprises a first row transition metals,or solid solutions, doped or modified compositions, or nonstoichiometricor disordered forms of such olivines. Taking the compound LiFePO₄ forillustrative example, the density of olivine LiFePO₄ is 3.6 g/cm³ andits formula weight is 157.77 g/mole. The concentration of Fe per literof the solid olivine is therefore: (3.6/157.77)×1000 cm³/liter=22.82molar. Even if present in a suspension diluted substantially by liquid,the molar concentration far exceeds that of typical redox electrolytes.For example, a 50% solids slurry has 11.41M concentration, exceedingeven highly concentrated vanadium flow battery electrolytes, and this isachieved without any acid additions.

In some embodiments, a positive electrode flowable redox composition inwhich the electrochemically active solid compound forming the particlesis LiCoO₂, the density is 5.01 g/cm³ and the formula weight is 97.874g/mole. The concentration of Co per liter is: (5.01/97.874)×1000cm³/liter=51.19 molar. The energy density of such semi-solid slurries isclearly a factor of several higher than that possible with conventionalliquid catholyte or anolyte solutions.

In some embodiments, a suspension of graphite in the liquid, which mayserve as a negative electrode flowable redox composition, is used. Inoperation, graphite (or other hard and soft carbons) can intercalatelithium. In graphite the maximum concentration is about LiC₆. Sincegraphite has a density of about 2.2 g/cm³, and the formula weight ofLiC₆ is 102.94 g/mole, the concentration of Li per liter of LiC₆ is:(2.2/102.94)×1000=21.37 molar. This is again much higher thanconventional redox flow battery anolytes.

Furthermore, the nonaqueous batteries can have cell working voltagesthat are more than twice as high as some aqueous batteries, where thevoltage can be limited to 1.2-1.5V due to the limitation of waterhydrolysis at higher voltage. By contrast, use of LiFePO₄ with graphitein a semi-solid redox flow cell provides 3.3V average voltage, andLiCoO₂ with graphite provides 3.7V average voltage. Since the energy ofany battery is proportional to voltage, the batteries using solidsuspension or condensed ion-supporting liquid redox flow compositionshave a further improvement in energy over conventional solution-basedredox flow cells.

Thus a non-aqueous semi-solid redox flow cell can provide the benefitsof both redox flow batteries and conventional lithium ion batteries byproviding for a higher cell voltage and for flow battery electrodes thatare much more energy dense than redox flow batteries by not beinglimited to soluble metals, but rather, comprising a suspension of solidor liquid electrode-active materials, or in the case of dense liquidreactants such as liquid metals or other liquid compounds, the flowbattery electrolyte may comprise a significant fraction or even amajority of the liquid reactant itself. Unlike a conventional primary orsecondary battery, the total capacity or stored energy may be increasedby simply increasing the size of the reservoirs holding the reactants,without increasing the amount of other components such as the separator,current collector foils, packaging, and the like. Unlike a fuel cell,such a semi-solid redox flow battery is rechargeable.

Amongst many applications, the semi-solid and condensed ion-supportingliquid redox flow batteries can be used to power a plug-in hybrid (PHEV)or all-electric vehicle (EV). Currently, for markets where the dailydriving distance is long, such as the U.S. where the median dailydriving distance is 33 miles, PHEVs are an attractive solution becausewith daily charging a battery that supplies 40 miles of electric range(PHEV40) is practical. For a car weighing about 3000 lb this requires abattery of approximately 15 kWh of energy and about 100 kW power, whichis a battery of manageable size, weight, and cost.

However, an EV of the same size for the same driving pattern generallywill require longer range, such as a 200 mile driving distance betweenrecharges, or 75 kWh, in order to provide an adequate reserve of energyand security to the user. Higher specific energy batteries are needed tomeet the size, weight and cost metrics that will enable widespread useof EVs. The semi-solid and condensed ion-supporting liquid redox flowbatteries can enable practical low cost battery solutions for suchapplications. The theoretical energy density of the LiCoO₂/carbon coupleis 380.4 Wh/kg. However, high power and high energy lithium ionbatteries based on such chemistry provide only about 100-175 Wh/kg atthe cell level, due to the dilution effects of inactive materials.Providing a 200 mile range, which is equivalent to providing 75 kWh ofenergy, requires 750-430 kg of current advanced lithium ion cells.Additional mass is also required for other components of the batterysystem such as packaging, cooling systems, the battery managementsystem, and the like.

Considering the use of conventional lithium ion batteries in EVs, it isknown that specific energy is more limiting than power. That is, abattery with sufficient energy for the desired driving range willtypically have more than enough power. Thus the battery system includeswasted mass and volume that provides unneeded power. The semi-solid orcondensed ion-supporting liquid redox flow battery can have a smallerpower-generating portion (or stack) that is sized to provide thenecessary power, while the remaining, larger fraction of the total masscan be devoted to the high energy density positive and negativeelectrode redox flow compositions and their storage system. The mass ofthe power-generating stack is determined by considering how much stackis needed to provide the approximately 100 kW needed to operate the car.Lithium ion batteries are currently available that have specific powerof about 1000-4000 W/kg. The power generated per unit area of separatorin such a battery and in the stacks of the flowable redox cell issimilar. Therefore, to provide 100 kW of power, about 25-100 kg of stackis needed.

The remainder of the battery mass may come predominantly from thepositive and negative electrode flowable redox compositions. As thetheoretical energy density for the LiCoO₂/carbon couple is 380.4 Wh/kg,the total amount of active material required to provide 75 kWh of energyis only 197 kg. In flow batteries the active material is by far thelargest mass fraction of the positive and negative electrode flowableredox compositions, the remainder coming from additives and liquidelectrolyte phase, which has lower density than the ion storagecompounds. The mass of the positive and negative electrode flowableredox compositions needed to supply the 75 kWh of energy is only about200 kg.

Thus, including both the stack mass (25-100 kg) and the positive andnegative electrode flowable redox composition mass (200 kg), asemi-solid redox flow battery to supply a 200 mile range may weigh 225to 300 kg mass, much less than the mass (and volume) of advanced lithiumion batteries providing the same range. The specific energy of such asystem is 75 kWh divided by the battery mass, or 333 to 250 Wh/kg, abouttwice that of current lithium cells. As the total energy of the systemincreases, the specific energy approaches the theoretical value of 380.4Wh/kg since the stack mass is a diminishing fraction of the total. Inthis respect the rechargeable lithium flow battery has different scalingbehavior than conventional lithium ion cells, where the energy densityis less than 50% of the theoretical value regardless of system size, dueto the need for a large percentage of inactive materials in order tohave a functioning battery.

Thus in one set of embodiments, a rechargeable lithium ion flow batteryis provided. In some embodiments, such a battery has a relatively highspecific energy at a relatively small total energy for the system, forexample a specific energy of more than about 150 Wh/kg at a total energyof less than about 50 kWh, or more than about 200 Wh/kg at total energyless than about 100 kWh, or more than about 250 Wh/kg at total energyless than about 300 kWh.

In another set of embodiments, a redox flow device uses one or morereference electrode during operation to determine the absolute potentialat the positive and negative current collectors, the potentials beingused in a feedback loop to determine the appropriate delivery rate ofpositive and negative electrode flowable redox compositions. Forexample, if the cathodic reaction is completing faster than the anodicreaction, the cell will be “cathode-starved” and greater polarizationwill occur at the positive electrode. In such an instance, detection ofthe cathode potential will indicate such a condition or impendingcondition, and the rate of delivery of positive electrode flowable redoxcomposition can be increased. If the redox flow cell is being used athigh power, and both cathode and anode reactions are completing andresulting in a fully discharged or charged state at the instant flowrates, this too can be detected using the current collector potentials,and the rates of both positive and negative electrode flowable redoxcompositions are increased so as to “match” the desired current rate ofthe cell.

More than one reference electrode may be used in order to determine thepositional variation in utilization and completeness of electrochemicalreaction within the flow battery. Consider for example a planar stackwherein the positive and negative electrode flowable redox compositionsflow parallel to the separator and electrodes, entering the stack at oneend and exiting at the other. Since the cathode-active and anode-activematerials can begin to charge or discharge as soon as they are inelectrical communication, the extent of reaction can differ at theentrance and the exit to the stack. By placing reference electrodes atmore than one position within the stack and within the cell, thenear-instantaneous state of the cell with respect to state of charge ordischarge and local polarization can be determined. The operatingefficiency, power and utilization of the cell can be optimized by takinginto account the voltage inputs from the reference electrodes andaltering operating parameters such as total or relative flow rate ofcatholyte and anolyte.

The reference electrodes may also be placed elsewhere within the flowdevice system. For example, having reference electrodes in the positiveand negative electrode flowable redox composition storage tanks, orhaving a separate electrochemical cell within the storage tanks, thestate of charge and discharge of the positive and negative electrodeflowable redox compositions in the tank can be monitored. This also canbe used as input to determine the flow rate of the semi-solidsuspensions when operating the battery in order to provide necessarypower and energy. The position of the reference electrode permits thedetermination of the local voltage in either the anolyte, catholyte, orseparator. Multiple reference electrodes permit the spatial distributionof voltage to be determined. The operating conditions of the cells,which may include flow rates, can be adjusted to optimize power densityvia changes in the distribution of voltage.

In some embodiments, the semi-solid redox flow cell is a nonaqueouslithium rechargeable cell and uses as the reference electrode a lithiumstorage compound that is lithiated so as to produce a constant potential(constant lithium chemical potential) over a range of lithiumconcentrations. In some embodiments the lithium-active material in thereference electrode is lithium titanate spinel or lithium vanadium oxideor a lithium transition metal phosphate including but not limited to alithium transition metal olivine of general formula Li_(x)M_(y)PO₄ whereM comprises a first row transition metal. In some embodiments thecompound is LiFePO₄ olivine or LiMnPO₄ olivine or mixtures or solidsolutions of the two.

Example 1 Semi-Solid Lithium Redox Flow Battery

An exemplary redox flow cell 200 for a lithium system is shown in FIG.2. In this example, the membrane 210 is a microporous membrane such as apolymer separator film (e.g., Celgard™ 2400) that prevents cathodeparticles 220 and anode particles 230 from crossing the membrane, or isa solid nonporous film of a lithium ion conductor. The negative andpositive electrode current collectors 240, 250 are made of copper andaluminum, respectively. The negative electrode composition includes agraphite or hard carbon suspension. The positive electrode compositionincludes LiCoO₂ or LiFePO₄ as the redox active component. Carbonparticulates are optionally added to the cathode or anode suspensions toimprove the electronic conductivity of the suspensions. The solvent inwhich the positive and negative active material particles are suspendedis an alkyl carbonate mixture and includes a dissolved lithium salt suchas LiPF₆. The positive electrode composition is stored in positiveelectrode storage tank 260, and is pumped into the electroactive zoneusing pump 265. The negative electrode composition is stored in negativeelectrode storage tank 270, and is pumped into the electroactive zoneusing pump 275. For the carbon and the LiCoO₂, the electrochemicalreactions that occur in the cell are as follows:

Charge: xLi+6xC→LiC₆ LiCoO₂→Li⁺+Li_(1-x)CoO₂Discharge: xLiC₆→xLi+6xC xLi⁺+Li_(1-x)CoO₂→LiCoO₂

Example 2 Semi-Solid Nickel Metal Hydride Redox Flow Battery

An exemplary redox flow cell for a nickel system is shown in FIG. 3. Inthis example, the membrane 310 is a microporous electrolyte-permeablemembrane that prevents cathode particles 320 and anode particles 330from crossing the membrane, or is a solid nonporous film of a proton ionconductor, such as Nafion. The negative and positive electrode currentcollectors 340, 350 are both made of carbon. The negative electrodecomposition includes a suspension of a hydrogen absorbing metal, M. Thepositive electrode composition includes NiOOH as the redox activecomponent. Carbon particulates are optionally added to the cathode oranode suspensions to improve the electronic conductivity of thesuspensions. The solvent in which the positive and negative activematerial particles are suspended is an aqueous solution containing ahydroxyl generating salt such as KOH. The positive electrode compositionis stored in positive electrode storage tank 360, and is pumped into theelectroactive zone using pump 365. The negative electrode composition isstored in negative electrode storage tank 370, and is pumped into theelectroactive zone using pump 375. The electrochemical reactions thatoccur in the cell upon discharge are as follows (the reactions uponcharging being the reverse of these):

Discharge: xM+yH₂O+ye⁻→M_(x)H_(y)+yOH⁻ Ni(OH)₂+OH⁻→NiOOH+H₂O+e⁻

Example 3 Reference Electrode Monitored Redox Flow Battery

An exemplary redox flow battery using a reference electrode to optimizecell performance is shown in FIG. 4. The cell includes two membranes410, 415. Reference electrodes 420, 425, 430 are positioned between thetwo membranes 410, 415 on a face opposite that of the electroactivezones 440, 445 where positive electrode redox flow composition 442 andnegative electrode redox flow composition 447 flow, respectively. Thecell also includes negative and positive current collectors 450, 460,respectively.

The potential at each reference electrode 420, 425 and 430 can bedetermined and are assigned a value of φ₁, φ₂ and φ₃, respectively. Thepotentials at the working electrodes (current collectors) 450, 460 canalso be determined and are assigned a value of W₁ and W₂, respectively.The potential differences of the cell components can be measured asfollows:

(W₁−W₂)=cell voltage

(W₂−φ₃)=potential at cathode

(W₁−φ₃)=potential at anode

(φ₃−φ₂) or (φ₂−φ₁)=extent of reaction as redox compositions flow alongstack.

In this example, three reference electrodes are used within the powergenerating stack (electroactive zone) in order to determine whether theflow rates of the positive and negative electrode redox flowcompositions are at a suitable rate to obtain a desired power. Forexample, if the flow rate is too slow during discharge, the positive andnegative electrode redox flow compositions fully discharge as the enterthe stack and over most of their residence time in the stack there isnot a high chemical potential difference for lithium. A higher flow rateallows greater power to be obtained. However, if the flow rate is toohigh, the active materials may not be able to fully charge or dischargeduring their residence time in the stack. In this instance the flow rateof the slurries may be slowed to obtain greater discharge energy, or oneor more slurries may be recirculated to obtain more complete discharge.In the instance of charging, too high a flow rate prevents the materialsfrom fully charging during a single pass, and the stored energy is lessthan the system is capable of, in which case the slurry flow rate may bedecreased, or recirculation used, to obtain more complete charging ofthe active materials available.

Example 4 Preparing Partially Delithiated, Jet-Milled Lithium CobaltOxide

Lithium cobalt oxide powder was jet-milled at 15,000 RPM to produceparticles with an average diameter of 2.5 microns. A 20 g sample ofjet-milled lithium cobalt oxide was chemically delithiated by reactingwith 2.5 g of nitronium tetrafluoroborate in acetonitrile over 24 hours.The delithiated Li_(1-x)CoO₂, having also a higher electronicconductivity by virtue of being partially delithiated, is used as theactive material in a cathode semi-solid suspension.

Example 5 Preparing a Copper Plated Graphite Powder

Commercial grade mesocarbon microbead (MCMB 6-28) graphitic anode powderwas partially coated with, 3.1% by weight, metallic copper via anelectroless plating reaction. MCMB (87.5 g) was stirred successively inthe four aqueous solutions listed in Table 1. Between each step, thepowder was collected by filtering and washed with reagent grade water.In the final solution, a concentrated solution of sodium hydroxide wasadded to maintain a pH of 12. Increasing the concentrations of thespecies in solution 4 would yield more copper rich powders. Powders withweight fractions 1.6%, 3.1%, 8.6%, 9.7%, 15%, and 21.4% copper werecharacterized by preparing slurries as described in Example 7, andtesting the slurries as described in Example 8. The cycling performanceincreased and capacity increased with copper plating weight percents asillustrated in FIG. 5.

TABLE 1 Four aqueous solutions used to treat MCMB. Solution ChemicalConcentration (M) 1 (1 hr) Nitric Acid 4.0 2 (2 hr) Stannous Chloride0.10 Hydrochloric Acid 0.10 3 (2 hr) Palladium Chloride 0.0058Hydrochloric Acid 0.10 4 (0.5 hr) Copper Sulfate 0.020 EDTA 0.050Formaldehyde 0.10 Sodium Sulfate 0.075 Sodium Formate 0.15 PolyethyleneGlycol 0.03 Sodium Hydroxide Maintain at pH 12

Example 6 Preparing a Cathode Slurry

A suspension containing 25% volume fraction of delithiated, jet-milledlithium cobalt oxide, 0.8% volume fraction of Ketjen Black, and 74.2%volume fraction of a standard lithium ion battery electrolyte wassynthesized. A stable cathode suspension was prepared by mixing 8.9 g ofdelithiated, jet-milled lithium cobalt oxide with 0.116 g of KetjenBlack carbon filler. The mixed powder was suspended in 5 mL ofelectrolyte and the suspension was sonicated for 20 minutes. Such asuspension was stable (i.e., there was no observable particle settling)for at least 5 days. The conductivity of such a suspension was measuredto be 0.022 S/cm in an AC impedance spectroscopy measurement. Suchslurries were tested in static and flowing cells as described in laterExamples. Experimentation with the relative proportions of theconstituents of the slurries showed that higher volume fractions oflithium cobalt oxide, which increase the storage capacity of thesuspension, can be made. Increasing the volume fraction of solids in thesuspension also increased the viscosity of the semi-solid suspensions.Higher volume fractions of Ketjen carbon particles increased suspensionstability and electronic conductivity, but also the slurry viscosity.Straightforward experimentation was used to determine volume fractionsof lithium cobalt oxide and Ketjen carbon that produce slurries ofsuitable viscosity for device operation.

Example 7 Preparing an Anode Slurry

A suspension containing 40% volume fraction of graphite in 60% volumefraction of a standard lithium ion battery electrolyte was synthesizedby mixing 2.88 g of copper plated graphite (3.1 wt % copper) with 2.0 mLof electrolyte. The mixture was sonicated for 20 minutes. Theconductivity of the slurry was 0.025 S/cm. Higher copper loadings on thegraphite was observed to increase the slurries' viscosity.

Example 8 Static Half Cell Tests on Cathode and Anode Slurries

Semi-solid suspension samples, as described in Examples 6 and 7, werecharged and discharged electrochemically against a lithium metalelectrode in an electrochemical cell where the suspension was static.The cathode or anode slurry was placed in a metallic well which alsoacted as the current collector. The well and current collectors weremachined from aluminum and copper for the cathode and anode,respectively. The wells holding the slurries had cylindrical shape 6.3mm in diameter and depths ranging from 250-800 μm. A Celgard 2500separator film separated the slurry from a lithium metal counterelectrode, and an excess of electrolyte was added to the gaps in thecell to ensure that the electrochemically tested materials remainedwetted with electrolyte. Testing was conducted in an argon-filledglovebox. A representative plot of voltage as a function of chargingcapacity for the cathode slurry half-cell is shown in FIG. 6. Arepresentative plot of the cathode discharge capacity vs. cycle numberis shown in FIG. 9. A representative plot of voltage as a function ofcharging capacity for the anode slurry half-cell is shown in FIG. 7.Both anode and cathode behaved electrochemically in a manner similar totheir solid (unsuspended) counterparts. Example capacity measurementsare shown in Table 2.

TABLE 2 Example capacity measurements. Specific Capacity in SpecificVolumetric mAh per Capacity in Capacity in gram of mAh per mAh per MCMBor gram of mL of Slurry Material LiCoO₂ Slurry Slurry MCMB with 0 wt %96 51 85 deposited Cu,¹ 40 vol % anode powder in electrolyte MCMB with3.1 wt % 344 179 300 Cu,² 40 vol % anode powder in electrolyte MCMB with15 wt % 252 123 219 Cu¹ 40 vol % anode powder in electrolyte MCMB with21.4 wt % 420 190 354 Cu,³ 40 vol % anode powder in electrolyte 26 vol %LiCoO₂, 0.8 97 56 127 vol % Ketjen Carbon Black in electrolyte⁴¹Capacity calculated from the 2^(nd) cycle discharge in a C/20galvanostatic cycling experiment between 0.01 V and 0.6 V versus Limetal; ²Capacity calculated from the 2^(nd) cycle discharge in a C/20CCCV charge, C/20 galvanostatic discharge cycling experiment between0.01 V and 1.6 V versus Li metal; ³Capacity calculated from the 2^(nd)cycle discharge in a C/20 galvanostatic cycling experiment between 0.01V and 1.6 V versus Li metal; ⁴Capacity calculated from 2^(nd) dischargein a C/3 galvanostatic cycling experiment between 4.4 V and 2 V.

Example 9 Static Cell Tests of Full Lithium Ion Cell Using Cathode andAnode Semi-Solid Suspensions

Cathode and anode slurries, as described in Examples 6 and 7, werecharged and discharged electrochemically against each other in a static,electrochemical cell. The cathode and anode slurries were each placed inmetallic wells/current collectors of the dimensions described in Example8. The wells/current collectors were made of aluminum and copper for thecathode and anode, respectively. A Celgard 2500 film separated the twoslurries in the cell. The cathode and anode suspensions were charged anddischarged relative to each other repeatedly under potentiostatic andgalvanostatic conditions, with galvanostatic testing being done atC-rates ranging from C/20 to C/10. A representative plot of voltage as afunction of time is shown in the lower panel in FIG. 8. Thecorresponding charge or discharge capacity is shown in the upper panelin FIG. 8. In this test, the cell was charged under potentiostaticconditions, holding the cell voltage at 4.4V, while the charge capacitywas monitored. The rate of charging is initially high, then diminishes.The cell was then galvanostatically discharged at a C/20 rate. Thecapacity obtained in the first discharge is ˜3.4 mAh, which is 88% ofthe theoretical capacity of the anode in the cell. There is an excess ofcathode in this cell which is therefore not fully utilized.

Example 10 Lithium Titanate Spinel Anode Suspension

Lithium titanate spinel, which may have a range of Li:Ti:O ratios andalso may be doped with various metals or nonmetals, and of which anon-limiting composition is Li₄Ti₅O₂, intercalates lithium readily at athermodynamic voltage near 1.5V with respect to Li/Li⁺, and increases inits electronic conductivity as Li is inserted due to the reduction ofTi⁴⁺ to Ti³⁺. A 5 g sample of lithium titanate spinel powder is mixedwith 100 mg of Ketjen Black and suspended in 10 mL of a standard lithiumion battery electrolyte, and the suspension is sonicated for 20 minutes.Such a suspension does not separate into components for at least 48hours. This suspension was charged and discharged in a lithium half-cellas described in Example 8. FIG. 10 shows the galvanostatic lithiuminsertion and extraction curves for the suspension at a relatively highC/1.4 rate. During the lithium insertion step, the average voltage isvery near the thermodynamic voltage of 1.55V, while upon extraction theaverage voltage is somewhat higher.

Example 11 Flowing Half Cell Tests on Cathode and Anode Slurries

Samples, as described in Examples 6 and 7, were charged and dischargedelectrochemically against a lithium metal electrode in a flowing,electrochemical cell. The cathode or anode slurry was pumped into ametallic channel of defined geometry, which acted as the currentcollector. The current collectors were aluminum and copper for thecathode and anode, respectively. Channels were 5 mm in diameter, 50 mmin length, and had a depth of 500 gm. A porous PVDF sheet (pore size:250 μm), sandwiched between 2 Celgard 2500 separator films, addedmechanical strength. In between the two separator films, separated fromthe slurries, was a lithium metal reference electrode attached to acopper wire and electrically isolated from both current collectors. Anexcess of liquid electrolyte was added to the gaps in the device toensure that the electrochemically active components remained immersed inliquid electrolyte. Testing was conducted in an argon-filled glove box.The slurry in the channel was charged and discharged at rates rangingfrom C/20 to C/5. During charging, uncharged slurry was mechanicallypumped into the test cell to replace that which had been fully chargedin the channel. The charged slurry was pumped out of the cell and storeduntil the end of the charge. For discharging, the cell was run inreverse, both electrochemically and mechanically. New volume of slurrywas pumped into the test cell as the volume in the cell was fullydischarged. The volume of discharged suspension was pumped out of thecell and stored until the end of the discharge.

Example 12 Flowing Full Cell Tests on Cathode and Anode Slurries

Cathode and anode slurries, as described in Examples 3 and 4, werecharged and discharged electrochemically in concert in a flowing,electrochemical cell. The cathode or anode slurry was pumped into ametallic channel, the channel material also acting as the currentcollector. The current collectors were aluminum and copper for thecathode and anode, respectively. Channels were 5 mm in diameter, 50 mmin length, and had a depth of 500 μm. A 250 μm perforated PVDF sheet,sandwich between 2 Celgard 2500 films, added mechanical strength andseparated one slurry channel from the other. A piece of lithium foilattached to a copper wire was also sandwiched between the separatorfilms and acted as a reference electrode. The slurries in the channelwere charged and discharged at rates ranging from C/20 to C/5. Usingperistaltic pumps, to which were attached elastomer tubing filled withcathode and anode slurries feeding the respective channels in theelectrochemical cells, the slurries were pumped through the channels.During charging, uncharged slurry was mechanically pumped into the testcell to replace that which was fully charged. For discharging, the cellwas run in reverse, both electrochemically and mechanically. The twoslurries were flowed independent of one another and the state of chargeof both anode and cathode slurries were monitored in real time using thelithium metal reference electrode. Several different modes of operationwere used. In one instance, one or both slurries were intermittentlypumped into the channels, the pumping stopped, and the slurries in thechannel were charged or discharged, following which the slurry in thechannel was displaced by fresh slurry and the process repeated. Inanother mode of operation, the slurries were pumped continuously, withthe residence time of each slurry in its respective channel beingsufficient for complete charge or discharge before exiting the channel.In yet another mode of operation, one or both slurries were pumpedthrough their respective channels at a rate too high for completecharging or discharging during the residence time, but the slurry wascontinuously circulated so that over time, all of the slurry in thesystem was either charged or discharged. In yet another mode ofoperation, the pumping direction of one or both slurries wasperiodically reversed during a charging or discharging step, causingmore slurry than the channel can accommodate at a given time to becharged or discharged.

It is recognized, of course, that those skilled in the art may makevarious modifications and additions to the processes of the inventionwithout departing from the spirit and scope of the present contributionto the art. Accordingly, it is to be understood that the protectionsought to be afforded hereby should be deemed to extend to the subjectmatter of the claims and all equivalents thereof fairly within the scopeof the invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. An energy storage system comprising: a positiveelectrode current collector, a negative electrode current collector, andan ion-permeable membrane separating said positive and negative currentcollectors, positioned and arranged to define a positive electroactivezone and a negative electroactive zone; wherein at least one of saidpositive and negative electroactive zone comprises a semi-solidcomposition comprising ion storage compound particles capable of takingup or releasing said ions during operation of the cell, and wherein theion storage compound particles have a polydisperse size distribution inwhich the finest particles present in at least 5 vol % of the totalvolume, is at least a factor of 5 smaller than the largest particlespresent in at least 5 vol % of the total volume.
 2. The energy storagesystem of claim 1, wherein the finest particles present in at least 5vol % of the total volume, is at least a factor of 7 smaller than thelargest particles present in at least 5 vol % of the total volume. 3.The energy storage system of claim 1, wherein the finest particlespresent in at least 5 vol % of the total volume, is at least a factor of10 smaller than the largest particles present in at least 5 vol % of thetotal volume
 4. The energy storage system of claim 1, wherein the ionstorage compound particles have a bidisperse size distribution in whichthe two maxima differ in size by at least a factor of
 5. 5. The energystorage system of claim 4, the particle size of the maxima for thelarger particles is at least 1 micrometer and preferably at least 10micrometers.
 6. The energy storage system of claim 1, wherein theparticles have morphology that is at least equiaxed.
 7. The energystorage system of claim 1, wherein the particle packing fraction is atleast 50 vol %, preferably at least 55 vol %, more preferably at least60 vol %, still more preferably at least 65 vol %, and still morepreferably at least 70 vol %.
 8. The energy storage system of claim 1,wherein the particle packing fraction is at least 55 vol %.
 9. Theenergy storage system of claim 1, wherein the particle packing fractionis at least 60 vol %.
 10. The energy storage system of claim 1, whereinthe particle packing fraction is at least, at least 70 vol %.
 11. Theenergy storage system of claim 1, further comprising a redox mediator.12. The energy storage system of claim 11, wherein the redox mediator issoluble in the semi-solid composition and comprises multiple oxidationstates.
 13. The energy storage system of claim 11, wherein the redoxmediator comprises a redox metal ion selected from iron, vanadium,chromium and zinc and mixtures thereof.
 14. The energy storage system ofclaim 1, wherein the semi-solid ion-storing redox composition furthercomprises an electronically conductive material.
 15. The energy storagesystem of claim 14, wherein the electronically conductive materialcomprises a conductive inorganic compound.
 16. The energy storage systemof claim 14, wherein the electronically conductive material forms apercolative conductive pathway.
 17. The energy storage system of claim14, wherein the electronically conductive material comprises anelectronically conductive polymer.
 18. The energy storage system ofclaim 14, wherein the electronically conductive material coats the ionstorage compound particles.
 19. The energy storage system of claim 1,wherein the one or both of the positive and negative current collectoris coated with an electronically conductive material.
 20. The energystorage system of claim 1, further comprising: at least one tankexternal to the energy storage system for delivering the semi-solidcomposition into the electroactive zone.
 21. The energy storage systemof claim 1, wherein one of said positive and negative electroactive zonecomprises a semi-solid composition comprising ion storage compoundparticles capable of taking up or releasing said ions during operationof the cell; and wherein one of said positive and negative electroactivezone comprises an aqueous redox solution capable of taking up orreleasing said ions during operation of the cell and an electronicallyconductive material.
 22. The energy storage system of claim 21, whereinthe electronically conductive material forms a percolative conductivepathway.
 23. A system comprising: (a) an energy storage systemcomprising: a positive electrode current collector, a negative electrodecurrent collector, and an ion-permeable membrane separating saidpositive and negative current collectors, positioned and arranged todefine a positive electroactive zone and a negative electroactive zone;wherein at least one of said positive and negative electroactive zonecomprises a semi-solid composition comprising ion storage compoundparticles capable of taking up or releasing said ions during operationof the cell, and wherein the ion storage compound particles have apolydisperse size distribution in which the finest particles present inat least 5 vol % of the total volume, is at least a factor of 5 smallerthan the largest particles present in at least 5 vol % of the totalvolume; and (b) at least one container external to the energy storagesystem for delivering the semi-solid composition into the electroactivezone.